A curated list of C good stuff. I give preference to free software for code, and sellers who aren't evil for physical resources.

This is released under the GNU Free Documentation License - its text is provided in the LICENSE file.

• Clang - A C compiler for LLVM. Supports C11. NCSA.
• GCC - Provides a C compiler as part of its compiler set. Supports C11 and OpenMP. GNU GPL3.
• LCC - A Retargetable Compiler for ANSI C. Nonfree (but liberal) license. License.
• PCC - A very old C compiler. Supports C99. Various licenses
• TCC - Tiny C Compiler; a small, fast C compiler. Supports C99 (except complex types). GNU GPL2.1.

• GnuTLS - A secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1.
• libgcrypt - A general-purpose cryptography library, with a range of available ciphers. GNU GPL2.1 and GNU LGPL2.1.
• LibreSSL - A BSD fork of OpenSSL. Various licenses, all semi-free.
• 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 - A modern and easy-to-use crypto library. Expat.

This lists databases and data stores with C APIs.

• Hiredis - A minimalistic client library for Redis. 3-clause BSD.
• LMDB - An ultra-fast, ultra-compact key-value embedded data store. newOpenLDAP.
• MariaDB - A robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. 3-clause BSD.
• PostgreSQL - A powerful object-relational database system. PostgreSQL licence
• Redis - An advanced key-value store. 3-clause BSD.
• SQLite - A self-contained, serverless, zero-configuration, transactional SQL database engine with a C interface. Public domain.
• UnQLite - A self-contained, serverless, zero-configuration, transactional NoSQL engine with a C interface. FreeBSD.

These are specifically fancier, IDE-type editors. If you want a programmer's text editor, and yours doesn't support C, I'd be quite surprised.

• Anjuta DevStudio - The GNOME IDE. GNU GPL3.
• CodeLite - A cross-platform IDE. GNU GPL2.1.
• Geany - A very small and fast IDE. GNU GPL2.1.
• KDevelop - The KDE IDE. GNU GPL2.1.

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. Apache2.0.
• C Algorithms - A collection of common algorithms and data structures for C. ISC.
• EFL - A large collection of useful data structures and functions. Various licenses, all free.
• GLib - A library of utility functions and structures, designed to be portable, efficient and powerful. GNU LGPL3.
• GIO - A modern and easy-to-use VFS API. GNU LGPL3.
• GObject - An object-oriented system and object model for C. GNU LGPL3.
• libnih - A lightweight library of C functions and structures. GNU GPL2.1.
• libU - A small library of basic utilities, including memory allocation, string manipulation and logging. 3-clause BSD.
• stb - A range of single-file libraries for C. Public domain.

• Allegro - A cross-platform, video game development and multimedia library. zlib.
• Corange - A game engine in pure C. FreeBSD.
• CSFML - A binding for SFML in C. zlib.
• FreeGLUT - An alternative to the OpenGL Utility Toolkit. Allows the creation and management of windows with OpenGL contexts. X11.
• GLFW - A multi-platform library for creating windows with OpenGL contexts. zlib.
• ioquake3 - The Quake3 engine, freed at last. GNU GPL2.1
• SDL - A cross-platform library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. zlib.

• klib - Small and lightweight implementations of common algorithms and data structures. Expat.
• SGLIB - Simple Generic Library; an implementation of a range of generic programming structures and idioms in C. GNU GPL3.

• Cogl - A GPU graphics and utilities API. GNU LGPL2.1.
• Clutter - A UI library based on OpenGL. GNU LGPL2.1.
• libjpeg - A library for reading and writing JPEG files. Free software license (check the source).
• libjpeg-turbo - A faster library for reading and writing JPEG files. Various licences.
• libxmi - A function library for rasterizing 2D vector graphics. GNU GPL3.
• mozjpeg - An improved JPEG encoder. 3-clause BSD.
• OpenGL - The industry standard for high-performance graphics, with a native C binding. Various licenses.
• SDL2 - Simple DirectMedia Layer is a cross-platform development library designed to provide low level access to audio, keyboard, mouse, joystick, and graphics hardware via OpenGL and Direct3D. zlib

These are specifically widget toolkits.

• Elementary - A widget toolkit. Part of the EFL. GNU LGPL2.1.
• GTK+ - A cross-platform widget toolkit. GNU LGPL2.1.
• IUP - Another cross-platform widget toolkit. Expat.
• Tk - A basic widget toolkit. Part of Tcl/Tk. Tcl/Tk License.
• XForms Toolkit - A widget toolkit designed for the XWindow system. GNU LGPL2.1.

• Jansson - A C library for encoding, decoding and manipulating JSON. Expat.
• jsmn - A minimalistic JSON parser. Expat.
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. GNU GPL3.
• YAJL - A 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.

• C: A Reference Manual 5E (online PDF)
• The C Programming Language 2E (online PDF)

• A tutorial on pointers
• Building C Projects
• C Primer Plus 5E (online PDF)
• The GNU C Programming Tutorial (online PDF)

• 8 gdb tricks you should know
• Diving into concurrency: trying out mutexes and atomics
• Introduction to OpenMP (video)
• memcpy vs memmove
• Object-Oriented Programming with ANSI C (online PDF)

• A quick tutorial on implementing and debugging malloc, free, calloc, and realloc
• Bit twiddling hacks
• Some dark corners of C
• The lost art of C structure packing
• Writing efficient C and C code optimization

• C Programming Language Certified Associate preparation course

• C Pocket Reference - A concise reference book for C99.

• C Programming: A Modern Approach - An excellent book to learn the basics from C from.
• Head First C - A 'head-first' style book for learning C.

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

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

• FFMPEG - A complete, cross-platform solution to record, convert and stream audio and video. GNU LGPL2.1, with some parts under GNU GPL2.1.
• GStreamer - A framework for audio and visual media. GNU LGPL2.1.
• lodepng - A simple PNG image decoder and encoder, requiring no other dependencies. 3-clause BSD.

• asnlc - A compiler of ASN.1 specifications into C source code. FreeBSD.
• GNU adns - An advanced, easy-to-use, asynch-capable DNS client library and utilities. GNU GPL3.
• GNU SASL - An implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GNU GPL3.
• GnuTLS - A secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1.
• gumbo-parser - An HTML5 parsing library in C99. Apache2.0.
• http-parser - An HTTP request/response parser. Expat.
• libcurl - A client-side URL transfer library, supporting a wide range of formats. curl license
• libev - Yet another event loop. FreeBSD.
• libevent - An event loop replacement for network servers. 3-clause BSD.
• libgss - Generic Security Service. GNU GPL3.
• libhttpd - A library to add basic web server capabilities to an application or embedded device. GNU GPL3.
• libidn - An implementation of the Stringprep, Punycode and IDNA specifications. GNU LGPL2.1.
• libmicrohttpd - A small C library that makes it easy to run an HTTP server as part of another application. GNU LGPL2.1.
• libsoup - A GNOME HTTP client/server library. Uses GObject. GNU LGPL2.1.
• LibreSSL - A BSD fork of OpenSSL. Various licenses, all semi-free.
• lwan - An experimental, scalable, high-performance HTTP server. GNU GPL2.1.
• mihl - A library for implementing an embedded HTTP server. FreeBSD.
• mongoose - Embedded web server for C. GNU GPL2.1.
• nanomsg - A C-based implementation of ZeroMQ. Expat.
• neon - An HTTP and WebDAV client library with a C interface. GNU LGPL3.
• onion - HTTP server library, designed to be easy to use. Apache2.0.
• 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 - A SIP implementation in C without additional dependencies. GNU GPL3.
• socket99 - A wrapper library for the BSD sockets API.
• Tox - A communication platform, designed to be a Skype-killer. GNU GPL3.
• ZeroMQ - High-performance message passing networking library. Implemented in C++, but has a C interface. GNU LGPL3.

• apophenia - A library for statistical and scientific computing. GNU GPL2.1 with some exceptions.
• 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
• FFTW - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GNU GPL2.1.
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GNU GPL3.
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GNU GPL2.1 and GNU LGPL2.1.
• GNU MPC - A library for complex number arithmetic. GNU LGPL3.
• GNU MPFR - A library for arbitrary-precision floating-point arithmetic. GNU LGPL2.1.
• GNU MPRIA - A portable mathematics library for multi-precision rational interval arithmetic. GNU GPL3.
• GSL - The GNU Scientific Library; a sophisticated numerical library. GNU GPL3.
• KISS FFT - A very simple fast Fourier transform library. 3-clause BSD.
• LAPACKE - A C interface to LAPACK. 3-clause BSD.
• Yeppp! - Very fast, SIMD-optimized mathematical library. 3-clause BSD.

• OpenMP - A set of C pragmas designed to allow for easy parallelization of code. Standard (licensing not applicable).
• pth - A portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GNU GPL3.
• pthreads - The POSIX thread library. Standard (no license applicable).
• TinyCThread - A portable, small implementation of the C11 threads API. zlib.

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

• PCRE - An implementation of regexes identical to that of Perl 5. 3-clause BSD.
• SLRE - Super Light Regular Expression library; a very small implementation of a subset of Perl regex syntax. GNU GPL2.1.
• TRE - A POSIX-compliant, feature-full regex library. FreeBSD.
• T-Rex - Another tiny regex library. zlib.

• c-capnproto - An implementation of the Cap'n Proto serialization protocol. Expat.
• libavro - A C implementation of the Avro data serialization system. Apache2.0.
• msgpackalt - A simple, light and fast binary serialization library. 3-clause BSD.
• protobuf-c - An implementation of Google Protocol Buffer in C. FreeBSD.
• xdr - External Data Representation; a standard for data serialization. Standard (no license applicable).

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. Various licenses (all free software).
• clib - Something of a package manager for C. Comes with a bunch of libraries of its own. Expat.
• gnulib - A collection of common GNU code. GNU GPL3.
• ulib - Another collection of source code. Expat.

This contains standard C libraries.

• Bionic - Google's C standard library, developed for Android. 3-clause BSD
• dietlibc - A 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 LGPL3.
• musl - A standard C library, compatible with POSIX 2008 and C11. Designed for static linking. Expat.

• CHEAT - A very simple unit testing framework. FreeBSD.
• Check - A unit testing framework for C. GNU LGPL2.1.
• cmocka - A unit testing framework with support for mock objects. Apache2.0.
• CUnit - Another unit testing framework for C. GNU LGPL2.1.
• minunit - Minimal 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.

• Flycheck - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GNU GPL3.
• Yasnippet - A template system, with C templates for common code snippets. GNU GPL3.

• Syntastic - Syntax checking and linting. Do What The Fuck You Want To license.
• YouCompleteMe - A 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.

• aimake - A build tool designed to avoid complex configurations. GNU GPL3.
• c99sh - Run C files using hash-bang. FreeBSD.
• GDB - The GNU Project debugger; a debugger for C. GNU GPL3.
• gprof - A performance analysis tool. Part of GNU binutils. GNU GPL3.
• libtool - A generic library support script. GNU GPL3.
• rr - A debugger that records non-deterministic executions to allow for deterministic debugging. FreeBSD.
• Valgrind - A 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.

• bfd - A library for manipulating binary object files. Part of GNU binutils. GNU GPL2.1.
• bstrlib - The Better String Library. 3-clause BSD or GNU GPL2.1.
• ccv - C-based/Cached/Core Computer Vision library; modern computer vision. 3-clause BSD.
• GNU FreeIPMI - An in-band and out-of-band IPMI implementation. GNU GPL3.
• GNU Libffcall - A collection of libraries for building foreign function interfaces. GNU GPL3.
• Hans Boehm GC - Garbage collection for C? Don't mind if I do! Various licenses, all free.
• ICU - International Components for Unicode; a library for Unicode support. ICU license.
• libavl - A library containing a range of self-balancing binary trees. GNU GPL2.1.
• libCello - A library introducing higher-level programming to C. 3-clause BSD.
• libffi - A portable foreign-function interface library. Expat.
• libgit2 - Pure C implementation of Git. GNU GPL2 with a linking exception.
• libgiconv - A text conversion library. GNU LGPL2.1.
• libPhenom - An eventing framework for building high-scalability and high-performance systems. Apache2.0.
• libunistring - A library for manipulating Unicode strings in C. GNU LGPL3.
• libuv - Cross-platform asynchronous I/O. Expat.
• ncurses - Coloured terminal UI library. GNU GPL3.
• nope.c - A C-language-based, ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GNU GPL2.1.
• Ragel - A DSL for state machines that compiles to C. GNU GPL3.
• 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.
• udpc - An implementation of the Universal Design Pattern. GNU GPL3.
• uthash - A hash table implementation, allowing existing structures to be stored in a hash table easily. 1-clause BSD

"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 - A stream-oriented XML parser. Expat.
• libxml2 - A standards-compliant, portable XML parser. Expat.