The ioquake3 community effort to continue supporting/developing id's
The flexible game engine
An OpenGL RTS game engine written in C
Orx: Data-Driven Game Engine
a UCI chess engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Cross-platform 2D and 3D game engine
Game engine for playing tomb raider 1 - 5 levels.
3D Game Engine
Small OpenGL based game enegine
A Simple Game Engine
3D game engine for C and C++
Tactical, turn-based game engine, that uses OpenGL and WinAPI
A DirectX based graphics/physics engine