Simple and portable (but not inflexible) GUI library in C that uses the native GUI technologies of each platform it supports.

A single-header ANSI C immediate mode cross-platform GUI library

nanomsg library

Instant Terminal Sharing

C library for building user interfaces

Unbound is a validating, recursive, and caching DNS resolver.

Wt, C++ Web Toolkit

CommonMark parsing and rendering library and program in C

A Simple cross-platform library for communicating with HID devices

A C library that may be linked into a C/C++ program to produce symbolic backtraces

Stepwise optimizations of DGEMM on CPU, reaching performance faster than Intel MKL eventually, even under multithreading.