RPG Maker 2000/2003 and EasyRPG games interpreter
Voxel game-creation platform with easy modding and game creation
The Ren'Py Visual Novel Engine
Multi Theft Auto is a game engine
Magic Another Game Engine
Game engine supporting many 3D/2D formats and graphic effects
Full featured MUD server with all the trimmings.
A conservative source port of Hexen that is identical to DOS
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
An online multiplayer turn based strategy game and board game engine.
C++ Text Adventure Engine
OSGET is the first open source mobile game engine template.
Free MMORPG Engine. Develop and host graphical browser-based games.