GemRB is a portable open-source implementation of the Infinity 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
Improved version of the X-Ray Engine, the game engine
KlayGE is a cross-platform open source game engine
Paintown is a 2d fighting game engine.
2D physics engine for games
WGFX 2D Game Engine