A curated list of C good stuff. This list contains only free software for code, and sellers who aren't evil for physical resources.

LOOKING FOR MAINTAINERS: This list is currently pretty much inactive. I'm looking for maintainers that can make it great again. See #26 for more info, but the process is simple: add or fix a couple of things in this list and you'll become a contributor to a 850+ stars list.

This list was previously maintained by @kozross. He decided to switch the list to a new platform, so I've decided to fork it so we could keep it on GitHub.

• Build Systems
• Compilers
• Compiler libraries
• Compression
• Crypto
• Database
• Deep Learning
• Documentation Generation
• Editors
• Embedded Systems
  • RTOS
• Environments
• Frameworks
• Game Programming
  • Engines
  • Resources
• Generic Programming
• Graphics
  • Graphic APIs
• Graphical User Interface
• Hashing
• Image Processing
• JSON
• Learning, Reference and Tutorials
  • Online
    • Reference
      • Language Standards
    • Beginner
    • Intermediate
    • Advanced
    • Self-study courses
  • Physical
    • Reference
    • Beginner
    • Intermediate
    • Advanced
• Macros
• Memory Allocators
• Multimedia
• Networking and Internet
  • Web Frameworks
• Numerical
• Parallel Programming
• Parsers
• Regex
• Serialization
• Source Code Collections
• Standard Libraries
• String Manipulation
• Testing
• Text Editor Extensions
  • Atom
  • Emacs
  • Vim
• Tools
• Utilities
• XML

These are tools to automate the building and testing of projects in C.

• aimake - Build tool designed to avoid complex configurations. GNU GPL3 or later
• Autoconf - Extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. Part of the Autotools. GNU GPL3 or later
• Automake - Tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Requires the use of Autoconf. Part of the Autotools. GNU GPL3 or later
• CMake - Tools for cross-platform building, testing, and packaging. 3-clause BSD
• Jam - Build system, designed to be easier than make. Understands C build rules implicitly. Jam License
• Libtool - Generic library support script. Part of the Autotools. GNU GPL3 or later
• Meson - Extremely fast, user-friendly build system. Based on Ninja. Apache 2.0
• Premake - Build script generator for toolsets like Visual Studio, Xcode, or GNU Make. 3-clause BSD
• SCons - An easier, more reliable and faster way to build software. MIT
• Ninja - Small, simple build system with a focus on speed. Apache 2.0
• Bazel - Build system for various operating systems and targets. Apache 2.0
• Buck - Build system created and used by Facebook. Apache 2.0
• Qbs - Modern build tool for software projects. LGPL3
• qmake - Build system included with the Qt Framework. GNU GPL3 with Qt Exception
• xmake - Cross-platform build utility based on lua. Apache 2.0

• Clang - C compiler for LLVM. Supports C11. NCSA
• CompCert - Fully-verified C compiler. Supports almost all of C89. GNU GPL2.1 or later
• GCC - Provides a C compiler as part of its compiler set. Supports C11 and OpenMP. GNU GPL3 or later
• PCC - Venerable C compiler. Supports C99. Various free licenses
• TCC - Tiny C Compiler; a small, fast C compiler. Supports C99 (except complex types). GNU LGPL2.1
• Intel SPMD - Compiler for a variant of the C language, for single program, multiple data programming. Various licenses
• ccache - Compiler cache designed to speedup recompilation. GNU GPL3 or later

Libraries for use in compilers, interpreters, and low-level based projects.

• yasm - Yasm Modular Assembler Project. 2-clause BSD

• libzip - A C library for reading, creating, and modifying zip archives. 3-clause BSD
• lzo - Very fast data compression library. GNU GPL2.1
• zlib - Massively-spiffy yet delicately-unobtrusive compression library. 3-clause BSD
• bzip2 - Patent free, high quality data compressor. 3-clause BSD
• lz4 - Fast Compression algorithm.
• snappy - Fast compression library (implementation in C++, native bindings to C).
• zstd - Fast real-time compression algorithm 3-clause BSD
• brotli - Generic lossless compression algorithm based on LZ77, Huffman coding and 2nd order context modelling MIT
• quicklz - Fast compression library. GNU GPL2.1
• pixz - Pixz (pronounced pixie) is a parallel, indexing version of xz. 2-clause BSD

• GnuTLS - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later
• libgcrypt - General-purpose cryptography library, with a range of available ciphers. GNU LGPL2.1or later (code) and GNU GPL2.1 or later (manual and tools)
• OpenSSL - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License
• libsodium - Modern and easy-to-use crypto library. Expat
• libtomcrypt - Fairly comprehensive, modular and portable cryptographic toolkit. Public Domain
• mbed TLS - Another crypto implementation for C. Apache 2.0
• libressl - Modernized fork of OpenSSL. Various Licenses

This lists databases and data stores with C APIs.

• BerkeleyDB - Library for a high-performance embedded database for key-value data. GNU AGPLv3
• Hiredis - Minimalistic client library for Redis. 3-clause BSD
• LMDB - Ultra-fast, ultra-compact key-value embedded data store. OpenLDAP License
• MariaDB - Robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. 3-clause BSD
• mongo-c-driver - High-performance client library for MongoDB. Apache 2.0
• PostgreSQL - Powerful object-relational database system. PostgreSQL licence
• recutils - Set of tools and a C library for accessing human-editable, plaintext database files called recfiles. GNU GPL3 or later
• Redis - Advanced key-value store. 3-clause BSD
• sophia - Modern, embeddable key-value database. FreeBSD
• SQLite - Self-contained, serverless, zero-configuration, transactional SQL database engine with a C interface. Public Domain
• UnQLite - Self-contained, serverless, zero-configuration, transactional NoSQL engine with a C interface. FreeBSD

• Darknet - Open source neural network framework written in C and CUDA. It is fast, easy to install, and supports CPU and GPU computation.

• Cxref - Generates documentation of C programs in either LaTeX, HTML, RTF or SGML. GNU GPL2.1
• DocOnce - Modestly-tagged markup language that can be used to generate a range of formats. 3-clause BSD
• Doxygen - The de-facto standard tool for generating C documentation from annotated sources. Can generate a large range of formats. GNU GPL2.1
• GTK-Doc - Tool for generating C documentation from annotated sources. Has support for the Autotools. GNU GPL2.1 (code) or GNU FDL1.1

These are specifically fancier, IDE-type editors. If you want a programmer's text editor, look elsewhere. Besides, whatever choice you make most likely supports C anyway.

• Anjuta DevStudio - The GNOME IDE. GNU GPL2.1
• Atom - Hackable text editor for the 21st century. MIT
• Code::Blocks - Extensible, configurable IDE supporting C. GNU GPL3
• CodeLite - Cross-platform IDE. GNU GPL2.1
• Eclipse - IDE written in Java. EPL
• Geany - Very small and fast IDE. GNU GPL2.1 or later
• KDevelop - The KDE IDE. GNU GPL2.1
• Qt Creator - Cross-platform IDE written with C++ and Qt, part of the Qt SDK. Supports Clang Code Model. GNU GPL3 with Qt exception
• Visual Studio Code - Code editing, redefined. Visual Studio Code.

These are projects that allow one to work with microcontrollers and other resource constrained architectures.

A real-time operating system (RTOS) is intended to serve real-time applications that process data with low latency delay.

• Amazon FreeRTOS - RTOS for microcontrollers that makes small, low-power edge devices easy to program. MIT
• ChibiOS - RTOS with complete development environment (HAL, drivers, support files, and tools). Various Licenses
• Contiki - Connect low-cost, low power microcontrollers to the Internet. 3-clause BSD
• Zephyr Project - Scalable real-time RTOS supporting multiple hardware architectures, optimized for resource constrained devices. Apache 2.0
• RIOT OS - Real-time multi-threading operating system that supports a range of devices that are typically found in the Internet of Things. LGPL-2.1

This is a list of technologies designed to bring Windows into the 21st century with respect to support for C.

• Cygwin - Designed to emulate a POSIX-compatible environment extensively under Windows. Various Licenses
• MinGW-w64 - Minimalist environment for C development on Windows with 64 bit support. Various Licenses

This section has big libraries that provide data structures and other stuff you expect of a 'modern' standard library.

• APR - Apache Portable Runtime; another library of cross-platform utility functions. Apache 2.0
• C Algorithms - Collection of common algorithms and data structures for C. ISC
• C Macro Collections - Header only, macro generated, generic and type-safe Collections in C. MIT
• CPL - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GNU GPL2.1
• EFL - Large collection of useful data structures and functions.
• GLib - Library of utility functions and structures, designed to be portable, efficient and powerful. GNU LGPL2.1
• GIO - Modern and easy-to-use VFS API. GNU LGPL2.1
• GObject - Object-oriented system and object model for C. GNU LGPL2.1
• libnih - Lightweight library of C functions and structures. GNU GPL2.1
• libU - Small library of basic utilities, including memory allocation, string manipulation and logging. 3-clause BSD
• PBL - Large library of utilities, featuring data structures, among other things. GNU LGPL2.1 or later (library) or GNU GPL2.1 or later (test code)
• qlibc - Simple and powerful C library, designed as a replacement for GLib while focusing on being small and light. qLib license (similar to FreeBSD)
• stb - Range of single-file libraries for C. Public Domain
• libsrt - Safe Real-Time library for C. 3-clause BSD

These are provided as examples of C game programming code.

• Corange - Game engine in pure C. FreeBSD
• Darkplaces - Modified version of the Quake2 engine. GNU GPL2.1
• ioquake3 - The Quake3 engine, freed at last. GNU GPL2.1
• Orx - Portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. zlib
• Quake - The Quake engine. GNU GPL2.1
• Quake2 - The Quake2 engine. GNU GPL2.1
• Spearmint - Engine designed for FPS games. GNU GPL3 or later

These are libraries of all sorts that are useful for game programming.

• Allegro - Cross-platform, video game development and multimedia library. zlib
• Chipmunk2D - Fast and lightweight 2D game physics library. Expat
• CSFML - Binding for SFML in C. zlib
• FreeGLUT - Alternative to the OpenGL Utility Toolkit. Allows the creation and management of windows with OpenGL contexts. X11
• GLFW - Multi-platform library for creating windows with OpenGL contexts. zlib
• libao - Cross-platform audio library with a wide variety of outputs. GNU GPL2.1 or later
• RetroArch - The reference frontend for libretro. GNU GPL3
• SDL and SDL2 - Cross-platform development library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. SDL2 is the most current version. zlib
• raylib - Simple and easy-to-use library to learn videogames programming. zlib

• klib - Small and lightweight implementations of common algorithms and data structures. MIT

• Cairo -2D graphics library. GNU LGPL2.1 or MPLv1.1
• Cogl - GPU graphics and utilities API. Expat
• Clutter - UI library based on OpenGL. GNU LGPL2.1
• giflib - Library for reading and writing gif images. MIT
• heman - Tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. Expat
• libcaca - ASCII renderer for terminal-based interfaces. WTFPLv2
• libimagequant - Small, portable library for high-quality conversion of RGBA images to 8-bit indexed colour images. FreeBSD
• libjpeg-turbo - Faster library for reading and writing JPEG files. Various Licenses
• libpng - The official PNG reference library. libpng license
• libxmi - Function library for rasterizing 2D vector graphics. GNU GPL3 or later
• mozjpeg - Improved JPEG encoder. 3-clause BSD

• OpenGL - Industry adopted 2D and 3D graphics API. More resources at awesome-opengl.
• OpenGL ES - Industry adopted 2D and 3D graphics API for mobile and embedded devices.
• OpenGL SC - Graphic and compute standard for industry requiring system safety certification.
• Vulkan - Explicit graphic and compute API for modern cross-platform development. More resources at awesome-vulkan.

These are specifically widget toolkits.

• GTK+ - Cross-platform widget toolkit. GNU LGPL2.1
• IUP - Another cross-platform widget toolkit. Expat
• Tk - Basic widget toolkit. Part of Tcl/Tk. Tcl/Tk License
• XForms Toolkit - Widget toolkit designed for the XWindow system. GNU LGPL2.1
• nuklear - Single-header ANSI C gui library. Public Domain
• libui - Simple and portable (but not inflexible) GUI library. MIT
• LCUI - Small C library for building user interfaces with C, XML and CSS. MIT

All hashing related libraries, cryptographic or not.

• jwHash - Fast hashtable implementation. Apache 2.0
• xxHash - Extremely fast non-cryptographic hash algorithm. 2-clause BSD
• libcrc - Multi platform CRC library. MIT
• murmur - C implementation of MurMur Hashing. Public Domain
• t1ha - Fast Positive Hash library. zlib

• libccv - Modern Computer Vision Library. 3-clause BSD

• Jansson - C library for encoding, decoding and manipulating JSON. Expat
• jsmn - Minimalistic JSON parser. Expat
• json-c - Library for working with JSON. Expat
• parson - Lightweight JSON library written in C. MIT
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. LGPL, any version
• YAJL - Fast C JSON streaming parser library. ISC

This is a list of resources for learning C programming in general, or something useful relating to C programming.

• SEI CERT C Coding Standard
• C FAQ - comp.lang.c Frequently Asked Questions
• Comparison of C/POSIX standard library implementations for GNU/Linux
• GNU C Reference Manual
• Robert Pike's notes on programming in C
• Draft ANSI C Rationale
• Draft C99 Rationale

• Draft ANSI C (C89) standard
• Draft C99 standard
• Draft C11 standard

• Tutorial on pointers
• Building C Projects
• C Programming Wikibook
• Introduction to 'fun' C
• Learning C with GDB
• POSIX Threads Programming tutorial (a little dated, but most of it is still valid and useful)
• The GNU C Programming Tutorial (online PDF)
• Templating in C

• 8 gdb tricks you should know
• 10 C99 tricks
• Diving into concurrency: trying out mutexes and atomics
• Introduction to OpenMP (video)
• OpenMP tutorial (for the OpenMP3 standard)
• memcpy vs memmove
• MPI tutorial
• Some unknown features or tricks in C language
• The lost art of C structure packing
• What a C programmer should know about memory
• What every C programmer should know about undefined behaviour

• Advanced metaprogramming in C
• Quick tutorial on implementing and debugging malloc, free, calloc, and realloc
• Bit twiddling hacks
• I do not know C
• Implementing smart pointers for the C programming language
• Inline functions in C
• Metaprogramming custom control structures in C
• Some dark corners of C
• Writing efficient C and C code optimization
• What every programmer should know about memory

• C Programming Language Certified Associate preparation course

• C: A Reference Manual 5E - Full reference book for C99.
• C Pocket Reference - Concise reference book for C99.
• The C Programming Language 2E - The original book on C, by its creators.

• C Primer Plus 6E - Complete tutorial on programming in C11.
• C Programming: A Modern Approach - Excellent book to learn the basics of C.
• Head First C - 'Head-first' style book for learning C.

• 21st Century C - Very good second programming book on C.
• Understanding and Using C Pointers - In-depth resource on pointers in C.
• ZeroMQ - Book for using ZeroMQ with C.

• Expert C Programming: Deep C Secrets - Interesting, in-depth and entertaining look at the innards of C.

• P99 - Suite of macros to implement advanced features like default function arguments, scope-bound resources, etc. (Requires C99) Q Public License

Implementations of memory allocators for various systems and platforms.

• jemalloc - General purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support, commonly used in production systems. FreeBSD
• dlmalloc - Doug Lea's malloc(3) implementation, useful for academic and research purposes. Public Domain
• Hoard - A Fast, Scalable, and Memory-efficient Malloc for Linux, Windows, Mac, and Solaris. GNU GPL2
• nedmalloc - An EXTREMELY FAST portable thread caching malloc(3) implementation written in C. GNU GPL2
• rpmalloc - Cross platform, lock-free memory allocator. Public Domain

• FFMPEG - Complete, cross-platform solution to record, convert and stream audio and video. GNU LGPL2.1 or later or GNU GPL2.1 or later (some parts)
• GStreamer - Framework for audio and visual media. GNU LGPL2.1
• lodepng - Simple PNG image decoder and encoder, requiring no other dependencies. 3-clause BSD
• libsoup - GNOME HTTP client/server library. Uses GObject. GNU LGPL2.1
• libmpv - Music-playing library. Compile with ./waf configure --disable-cplayer --enable-libmpv-shared to not have the music player. GNU GPL2.1 or later
• libsoundio - Library for cross-platform, real-time audio input and output. Has a range of back-ends. Expat

• asnlc - Compiler of ASN.1 specifications into C source code. FreeBSD
• czmq - High-level binding for ZeroMQ. MPL2.0
• GNU adns - Advanced, easy-to-use, asynch-capable DNS client library and utilities. GNU GPL3 or later
• GNU SASL - Implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GNU GPL3 or later
• GnuTLS - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later
• gumbo-parser - HTML5 parsing library in C99. Apache 2.0
• http-parser - HTTP request/response parser. Expat
• ldns - Library to simplify DNS programming. 3-clause BSD
• libcurl - Client-side URL transfer library, supporting a wide range of formats. curl license
• LibEtPan - Mail library providing an efficient network for IMAP, SMTP, POP and NNTP. 3-clause BSD
• libev - Yet another event loop. FreeBSD
• libuv - Cross-platform asynchronous I/O. Expat
• libevent - Event loop replacement for network servers. 3-clause BSD
• libgss - Generic Security Service. GNU GPL3 or later
• libhttpd - Library to add basic web server capabilities to an application or embedded device. GNU GPL2
• libidn - Implementation of the Stringprep, Punycode and IDNA specifications. GNU GPL3 or later
• libmicrohttpd - Small C library that makes it easy to run an HTTP server as part of another application. GNU LGPL2.1 or later
• libsagui - Sagui is a cross-platform C library which helps to develop web servers or frameworks. GNU LGPL3
• libvldmail - Your friendly e-mail address validation library. WTFPLv2
• lwan - Experimental, scalable, high-performance HTTP server. GNU GPL2.1
• mongoose - Embedded web server for C. GNU GPL2.1
• MQTT-C - Portable MQTT C client for embedded systems and PCs alike. MIT
• nanomsg - C-based implementation of ZeroMQ. Expat
• OpenSSL - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License
• oSip - SIP implementation in C without additional dependencies. GNU LGPLv2.1 or later
• pig - Linux packet crafting tool. GPL2
• s2n - C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache 2.0
• socket99 - C99 wrapper for the BSD sockets API. ISC
• Tox - Communication platform, designed to be a Skype-killer. GNU GPL3
• twitc - Mini C library for interacting with the Twitter OAuth API. Expat
• librg - Library for building simple and elegant cross-platform mmo client-server solutions. Apache 2.0
• dyad - Asynchronous networking library. MIT
• h2o - Optimized HTTP/1, HTTP/2 high performance server/library. MIT

• balde - Microframework for C based on GLib. GNU LGPLv2.1
• onion - C library to create simple HTTP servers and Web Applications. Apache 2.0
• kore - Easy to use, scalable and secure web application framework for writing web APIs in C.
• klone - KLone is a fully-featured, multiplatform, web application development framework.
• duda - Duda I/O is an event-driven and high performant web services framework written in C. Apache 2.0

• apophenia - Library for statistical and scientific computing. GNU GPL2.1
• ATLAS - Automatically Tuned Linear Algebra Software. 3-clause BSD
• BLAS - Basic Linear Algebra Subprograms; a set of routines that provide vector and matrix operations. BLAS license
• CDFLIB - Library with routines to evaluate cumulative density functions fo a variety of standard probability distributions. Also can compute one parameter of the CDF given the others. (No license given)
• cmathl - Pure-C Math library with a great variety of mathematical functions and CMake build support. Seeks to be close to C89/C90 compliant for portability. MIT
• Cuba - Library for multidimensional numerical integration. GNU LGPLv3
• FFTW - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GNU GPL2.1 or later
• FLINT - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GNU GPL2.1 or later
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GNU GPL3 or later
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GNU GPL2.1 and GNU LGPLv3
• GNU MPC - Library for complex number arithmetic. GNU LGPL3 or later
• GNU MPFR - Library for arbitrary-precision floating-point arithmetic. GNU LGPL3 or later or GNU LGPL2.1 or later (until version 2.4.x)
• GNU MPRIA - Portable mathematics library for multi-precision rational interval arithmetic. GNU GPL3 or later
• GSL - The GNU Scientific Library; a sophisticated numerical library. GNU GPL3
• igraph - Library for creating and manipulating large graphs. GNU GPL2
• KISS FFT - Very simple fast Fourier transform library. 3-clause BSD
• LAPACKE - C interface to LAPACK. 3-clause BSD
• PARI/GP - Computer algebra system for number theory; includes a compiler to C. GNU GPL2.1 or later
• PETSc - Suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. FreeBSD
• PROB - Library that handles various discrete and continuous probability density functions. GNU LGPL3
• Yeppp! - Very fast, SIMD-optimized mathematical library. 3-clause BSD

• cchan - Small library for channel constructs for inter-thread communication. Public Domain
• ck - Concurrency primitives, safe memory reclamation mechanisms and non-blocking data structures. FreeBSD
• mill - Go-style concurrency in C. X11
• libdill - Structured concurrency in C. X11
• MPICH - Another implementation of MPI. MPICH licence
• OpenMP - Set of C pragmas designed to allow for easy parallelization of code. 3-clause BSD
• OpenMPI - Message passing interface implementation. 3-clause BSD
• pth - Portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GNU GPL3 or later
• pthreads - The POSIX thread library.
• SLEPc - Software library for the solution of large, sparse eigenvalue problems on parallel computers. GNU LGPL3
• TinyCThread - Portable, small implementation of the C11 threads API. zlib

• hammer - Parser combinators for binary formats. GNU GPL2.1
• iniparser - Parser for .ini files. Expat
• libconfini - Yet another INI parser. GNU GPL3 or later
• libYAML - YAML 1.1 parser and emitter. Expat
• mpc - Parser combinator library. FreeBSD
• libucl - Universal configuration library parser. FreeBSD

"Some people, when confronted with a problem, think 'I know, I'll use regular expressions'. Now they have two problems." - Jamie Zawinski.

• PCRE - Implementation of regexes identical to that of Perl 5. 3-clause BSD
• regexp4 - Simple and complete implementation of regular expressions with its own sintax. GNU GPL3 or later
• SLRE - Super Light Regular Expression library; a very small implementation of a subset of Perl regex syntax. GNU GPL2.1
• sregex - Non-backtracking NFA/DFA-based Perl-compatible regex engine library. 3-clause BSD
• TRE - POSIX-compliant, feature-full regex library. FreeBSD

• binn - Binary serialization format meant to be compact, fast and easy-to-use. Apache 2.0
• c-capnproto - Implementation of the Cap'n Proto serialization protocol. Expat
• cmp - Implementation of the MessagePack serialization protocol. Expat
• libavro - C implementation of the Avro data serialization system. Apache 2.0
• mpack - Another implementation of the MessagePack serialization protocol. Expat
• netstring-c - Netstring encoder and decoder. Public Domain
• protobuf-c - Implementation of Google Protocol Buffer in C. FreeBSD
• xdr - External Data Representation; a standard for data serialization.

Check also: JSON, XML

This contains collections of small source code. If you want something big and integrated, check the Frameworks section.

• CCAN - Modelled after Perl's CPAN, this is a big collection of C code that does stuff. The full list is here.
• clib - Something of a package manager for C. Comes with a bunch of libraries of its own. Expat
• gnulib - Collection of common GNU code.
• libdjb - Collection of libraries doing various things. (Apparently) Public Domain

This contains standard C libraries.

• Bionic - Google's C standard library, developed for Android. 3-clause BSD
• dietlibc - C standard library designed for the smallest possible binaries. GNU GPL2.1
• glibc - The GNU C Library; an implementation of the C standard library. GNU LGPL2.1
• musl - Standard C library, compatible with POSIX 2008 and C11. Designed for static linking. Expat

• bstrlib - The Better String Library. 3-clause BSD and GNU GPL2.1
• ICU - International Components for Unicode; a library for Unicode support. ICU license
• libunistring - Library for manipulating Unicode strings in C. GNU LGPL3
• libgiconv - Text conversion library. GNU LGPL2.1 (library) or GNU GPL3 (*iconv* program)
• SDS - Simple Dynamic Strings; a library for handling C strings in a simpler way, but one that is compatible with normal C string functions. Available via clib. FreeBSD
• shoco - Compressor for small text strings. Expat
• smaz - Efficient string compression library. 3-clause BSD
• utf8.h - Single header utf8 string functions. Unlicense
• utf8proc - Small, clean library for processing UTF-8 Unicode data. License

• CHEAT - Very simple unit testing framework. FreeBSD
• Check - Unit testing framework for C. GNU LGPL2.1
• CMock - Mock/stub generator for C. Expat
• cmocka - Unit testing framework with support for mock objects. Apache 2.0
• Criterion - KISS, non-intrusive C test framework. Expat
• CUnit - Another unit testing framework for C. GNU LGPL2.0
• Cutest - Library for unit testing with memory leak detection (Linux, freeBSD and Windows). GPL2
• minunit - Minimal unit testing framework for C. Expat
• munit - Small but full-featured unit testing framework for C with no dependencies. MIT
• Unity - Simple unit testing framework for C. Expat

While practically any decent programmer's text editor supports C, there are some extensions that make it more pleasant. This is categorized by editor.

• linter-clang - Lint C code in Atom, using Clang. MIT
• linter-gcc - Lint C code in Atom, using GCC. MIT

• CEDET - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GNU GPL3 or later
• Flycheck - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GNU GPL3 or later
• Yasnippet - Template system, with C templates for common code snippets. GNU GPL3 or later

• Syntastic - Syntax checking and linting. Do What The Fuck You Want To license
• YouCompleteMe - Code completion engine for Vim. GNU GPL3

This is a list of useful programs to help you write and debug C code which are not editors, libraries or compilers.

• Artistic Style - Fast and small automatic source code formatter that supports C. GNU LGPL3
• address-sanitizer - Fast memory error detector. Apache 2.0
• biicode - Modern dependency manager for C. Expat
• c - Compile and execute C "scripts" in one go on the command line. Also has shebang support. Expat
• c99sh - Run C files using hash-bang. FreeBSD
• CBMC - C Bounded Model Checker; a tool for verification of array bounds, pointer safety and user-specified assertions. Original BSD
• cdecl - Online service to translate C declarations into English and vice versa. Public Domain
• cinclude2dot - Graphs include dependencies in a C project using Graphviz. Any GNU GPL version (due to underspecification in the file)
• Complexity - Tool for measuring the complexity of C source code. GNU GPL3 or later
• CScout - Source code analyzer and refactoring browser for collections of C programs. GNU GPL3
• DDD - Graphical front-end for a range of command-line debuggers. GNU GPL3 or later
• GDB - The GNU Project debugger; a debugger for C. GNU GPL3 or later
• Glade - RAD tool to enable quick development of GTK+ GUIs. GNU GPL2.1
• GMSL - GNU Make Standard Library; a collection of additional functionality for GNU Make. 3-clause BSD
• GNU Global - Source code tagging tool which works with C. GNU GPL3
• gprof - Performance analysis tool. Part of GNU binutils. GNU GPL3 or later
• Highlight - Converts source code to formatted text with nice highlighting. GNU GPL3
• include-what-you-use - Helps find unecessary inclusions and make suggestions for fixing them. Based on LLVM/Clang (and only works with it). NCSA
• indent - Formats C source code automatically to make it easier to read. Also converts from one style of source to another. GNU GPLv3 or later
• rr - Debugger that records non-deterministic executions to allow for deterministic debugging. FreeBSD
• tup - Very fast, file-based, cross-platform build system. GNU GPL2.1
• unifdef - Removes #ifdef and #if directives with their delimited text without touching any other part of the file. 3-clause BSD and FreeBSD
• Valgrind - Range of dynamic analysis tools, including a leak checker. GNU GPL2.1

This is a 'catch-all' category for anything that doesn't fit well anywhere else.

• ApeTagLibs - C library for working with APEv2 tags. Expat
• bfd - Library for manipulating binary object files. Part of GNU binutils. GNU GPL3 or later
• cf4ocl - The C Framework for OpenCL; a cross-platform object-oriented framework for developing and benchmarking OpenCL projects. GNU LGPL3 (library) or GNU GPL3 (project code)
• CommonMark - C implementation of the CommonMark spec. Various Licenses
• CException - C implementation of exceptions. Expat
• docopt.c - C implementation of a command-line option parser. Expat
• dyncall - Another foreign function interface library. Expat
• FANN - Fast Artifical Neural Network library; an implementation of neural networks. GNU GPL2.1
• Firm - C library that provides a graph-based intermediate representation, optimizations and assembly code generation suitable for use in compilers. Comes with an example C front-end under the same license. GNU LGPLv2.1
• Genann - Simple neural network library in C. Zlib
• gjrand - Library of random-number generation routines. GNU GPL2.1 or GNU GPLv3
• GNU FreeIPMI - In-band and out-of-band IPMI implementation. GNU GPL3
• GNU gperf - Perfect hash function generator, given a list of strings. Outputs C code. GNU GPL3 or later
• GNU Libffcall - Collection of libraries for building foreign function interfaces. GNU GPL3 or later
• gperftools - Collection of utilities for measuring and improving performance. 3-clause BSD
• huffandpuff - Minimal Huffman encoder and decoder. Public Domain
• kdtree - Simple library for working with KD-trees. 3-clause BSD
• Kitsune - Efficient, general-purpose framework for dynamic software updating. GNU LGPL3 or later
• libavl - Library containing a range of self-balancing binary trees. GNU GPL3 or later
• libbson - BSON utility library. Apache 2.0
• libCello - Library introducing higher-level programming to C. 3-clause BSD
• libffi - Portable foreign-function interface library. Expat
• libgit2 - Pure C implementation of Git. GNU GPL2 only, with a linking exception
• libimobiledevice - Cross-platform protocol library to communicate with iThings. GNU LGPLv2.1 or later (library) or GNU GPL2.1 or later (tools)
• libnfc - Platform-independent Near-Field Communication library. GNU LGPL3
• libPhenom - Deprecated, here for archival purposes. Eventing framework for building high-scalability and high-performance systems. Apache 2.0
• ncurses - Coloured terminal UI library. GNU GPL3 or later
• nope.c - C-language-based, ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GNU GPL2.1
• pbc - Protocol buffers library. Expat
• rabbitmq-c - Client library for RabbitMQ. Expat
• Ragel - DSL for state machines that compiles to C. GNU GPL2.1
• Tinyexpr - Tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. zlib
• uthash - Hash table implementation, allowing existing structures to be stored in a hash table easily. 1-clause BSD
• cpu_features - A cross platform C99 library to get cpu features at runtime. Apache 2.0
• pblog - Small, low overhead, structured logging library intended for logging formware events.Apache 2.0
• libelf - ELF object file access library in C. GNU GPL2.1
• mcpp - Portable C preprocessor. 2-clause BSD
• libusb - Generic access to USB devices. LGPL2.1
• alsa-lib - Userspace library to interact with ALSA. LGPL2.1
• Capstone - Dissasembly/disassembler framework. BSD 3-clause

"XML is crap. Really. There are no excuses. XML is nasty to parse for humans, and it's a disaster to parse even for computers. There's just no reason for that horrible crap to exist." - Linus Torvalds

• Expat - Stream-oriented XML parser. Expat
• libxml2 - Standards-compliant, portable XML parser. Expat
• mini-xml - Small XML reading and writing library. No dependencies aside from C standard library. Apache 2.0 with exceptions