The ioquake3 community effort to continue supporting/developing id's
The flexible game engine
Orx: Portable Game Engine
A cross-platform, open-source, pure C game engine for mobile game
Retro game engine for creating games like it's 1997
Game engine for playing tomb raider 1 - 5 levels.
A Simple Game Engine
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Reference implementation for the Doom game engine.