Zig game engine & graphics toolkit
A dead simple 2D game engine for Go
GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
Orx: Portable Game Engine
The multiplayer game engine
A game engine for console based text games
a UCI chess 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
Opensource Game Studio
Low-Level Game Engine for Retro Platforms
Flexible and easy-to-use 2d game engine
Multi-platform game engine similar to old Megaman with a stage editor
An online multiplayer turn based strategy game and board game engine.