fheroes2 is a recreation of Heroes of Might and Magic II game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
A cell-based puzzle game featuring classic and completely new puzzles
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
Go 3D Game Engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A generic 2D engine for any kind of game
2D Game Engine, based on OpenGL and focused on Isometric Games
Gomoku Game: Playing with Computer