Xash3D FWGS engine
The ioquake3 community effort to continue supporting/developing id's
A completely free, open-source, 2D game engine
Go bindings for raylib, a simple and easy-to-use library
A free 3d game for PC that does not limit the player. Fight,Build,Gain
starrydb is game server engine
Small OpenGL based game enegine
A complete 2D Game Engine with built in drag and drop editor
Quake III Arena GPL Source Release
Reference implementation for the Doom game engine.