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

QR decoder library

minimal cross-platform standalone C headers

List of single-file C/C++ libraries, with emphasis on clause-less licenses.

Dear ImGui: Bloat-free Graphical User interface for C++ with minimal dependencies

stb single-file public domain libraries for C/C++

A standalone and lightweight C library

A JavaScript implementation of various web standards, for use with Node.js