Header-only NES emulation library with an easy to use API. Include the header and instantiate the emulator with a handful of lines. Seems a fully featured NES emulator, capable of running even later games like SMB3. MIT licensed.
Pi-based FM Number Station w/ basic Speech Synthesis.
A free, online book on the C programming language. There is also a downloadable .pdf file of the contents of the e-book so you don't have to try to archive the whole website.
A small utility that searches for arbitrary strings in XOR or ROL encoded files. Automates the process of deciphering the files to do so.
An open source decompilation application that helps the user reverse engineering applications. Retargetable to different CPUs, so you can (say) reverse a SPARC binary on a PowerPC machine. Modular.
A frequently asked questions file about network programming in C on UNIX and its variants. Very handy!
Along with NIST SP800-22rev1 (statistical analysis of pseudo-random number generators), the Statistical Test Suite can be used to actually perform the tests outlined in this document. Runs under Windows, Linux, and Mac OSX. Written in C.
A list of tricks and methods for optimizing code written for the ATmel microcontrollers using GNU GCC.
Hammer is a parsing library written in C which aims to solve the language-theoretic security problem. It provides a parser which was designed from the ground up to be more secure as well as a selection of parsing back ends for the domain specific languages implemented thereby. It's also bit oriented, so you can use it to parse things which are ordinarily considered too fiddly (such as raw IP packets).
Bindings for other languages are available.
A massive list stored in a Github repository linking to free-as-in-beer ebooks about programming: Languages, platforms, tools, and more. Ideal for teaching classes, continuing education, reference, and getting a jump start on programming in school.
In response to the plethora of closed-source and API-only neural network and machine learning software out there the GNU Project has developed Gneural Network, a F/OSS framework which helps the developer build their own projects. Written in C, developed for portability. Has its own scripting language but can be referenced as a library from your own code.
Eucalyptus is a virtualization management stack which mimics the AWS web API, so infrastructure automation packages can use the EC2 protocol stack with it. Five functional components: Cloud Controller (web and API interface), Cluster Controller (manages virtual machines), Storage Controller (EBS workalike), Node Controller (manages the VMs and virtual networks), and Walrus (S3 workalike). Can be used on top of KVM so it requires the libvirt package. Written primarily in Java with some C.
mbed TLS (formerly PolarSSL) is an SSL implementation written from scratch designed for use in embedded applications and systems. The API was designed to make sense (unlike some other implementations I could mention) and the source code is written with readability in mind. Written in C as portably as possible. Modules are designed to be as loosely coupled as is feasible. opensource by default, but they do ask you to purchase commercial licenses if appropriate. Has a not-insignificant list of commercial and government users.
FOSS software that uses an RTL-SDR radio receiver as a cryptographic entropy generator. Pulls samples of RF noise, processes them, and seeds an RNG with it. Written in C.