ENet's purpose is to provide a relatively thin, simple and robust network communication layer on top of UDP (User Datagram Protocol). The primary feature it provides is optional reliable, in-order delivery of packets.

the Nothing itself nothings public domain versions of jpeg, png, ogg vorbis and zlib decompression. Also stb.h

GLFW is a free, Open Source, multi-platform library for opening a window, creating an OpenGL context and managing input. It is easy to integrate into existing applications and does not lay claim to the main loop.

The OpenGL Extension Wrangler Library (GLEW) is a cross-platform open-source C/C++ extension loading library. GLEW provides efficient run-time mechanisms for determining which OpenGL extensions are supported on the target platform. OpenGL core and extension functionality is exposed in a single header file. GLEW has been tested on a variety of operating systems, including Windows, Linux, Mac OS X, FreeBSD, Irix, and Solaris.

The axTLS embedded SSL project is a highly configurable client/server TLSv1 SSL library designed for platforms with small memory requirements. It comes with a small HTTP/HTTPS server and additional test tools. (License: BSD style)

musl is lightweight, fast, simple, free, and strives to be correct in the sense of standards-conformance and safety.

FLTK - Fast Light Toolkit is a cross-platform C++ GUI toolkit for UNIX/Linux (X11), Microsoft Windows, and MacOS X. FLTK provides modern GUI functionality without the bloat and supports 3D graphics via OpenGL and its built-in GLUT emulation.

libGDX desktop/Android/BlackBerry/iOS/HTML5 Java game development framework (licensed under Apache 2.0).

Real-Time Physics Simulation home of the open source Bullet Physics Library and physics discussion forums.

SDL
SDL Projects
SDL_image
SDL_mixer
SDL_net
SDL_ttf

The Khronos Group Inc. home to a variety of SDKs/libraries like:

• OpenGL for High Performance Graphics
• OpenGL ES for Embedded Accelerated 3D Graphics
• OpenSL ES for Embedded Audio Acceleration
• OpenCL for parallel programming of heterogeneous systems
• WebGL OpenGL ES 2.0 for the Web
• WebCL Heterogeneous parallel computing in HTML5 web browsers
• EGL an interface between Khronos rendering APIs such as OpenGL ES or OpenVG and the underlying native platform window system. It handles graphics context management, surface/buffer binding, and rendering synchronization and enables high-performance, accelerated, mixed-mode 2D and 3D rendering using other Khronos APIs.
• Among others.

Khronos OpenROAD is an animated video featuring all the royalty-free Khronos APIs working together in an open ecosystem.