A modern, cross-platform Quake game engine based on FitzQuake.
Game engine supporting many 3D/2D formats and graphic effects
C++ lightweight cross-platform game engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A 3D game engine in java.
3D game engine for C and C++
Game engine supporting many 3D/2D formats and graphic effects
The Lambda-Engine is a cross platform game engine.