GemRB is a portable open-source implementation of the Infinity Engine
Full featured MUD server with all the trimmings.
Game engine supporting many 3D/2D formats and graphic effects
A game engine for console based text games
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A generic 2D engine for any kind of game
Open source, moddable game engine for the "Polygon Love 2" game
Gomoku Game: Playing with Computer