GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
C++ lightweight cross-platform game engine
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
A cross platform Lua based game engine
Opensource Game Studio
Portable game engine with main focus on point-and-click adventures
scriptable game engine with built-in support for 2D graphics and sound
Multi-platform game engine similar to old Megaman with a stage editor
A generic 2D engine for any kind of game
Open Source 3D Game Engine
A 2D Adventure Game & Engine
TurboSphere is an easy to learn and powerful JS based game engine.
A 3D, fully concurrent game engine