GemRB is a portable open-source implementation of the Infinity Engine
...It was written to support pseudo-3D role playing games based on the Dungeons & Dragons ruleset (Baldur's Gate and Icewind Dale series, Planescape: Torment). It should run on GNU/Linux, Microsoft Windows, OS X, possibly other UNIX derivatives and more. It is licensed under the GPL.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A portable game engine for classic first person shooters such as DOOM, Heretic and Hexen. Lets you enjoy the original games using modern technology, including high-resolution OpenGL graphics, 3D models, and dynamic lighting effects.
Generic Collectible Card Game is a multiplayer multiplatform implementation of a card game engine. The card game engine is designed to be of general purpose core for several modules each defining the game specific behaviour.
Spring is a versatile 3D RTS game engine. Using extensively Lua for scripting game-specific code to make nearly every aspect of the engine customizable, from GUI, to unit AI, to pathfinding.