RTSish Game Engine
A free 3D game engine to create games and realtimes scenes for Windows
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
2D Game Engine
3D-Accelerated open-source DOOM engine
IRB is a game creation toolkit
sauerbraten with LuaJit2 scripting
MultiVice - is a fully customizable multiplayer modification for Grand
Karaoke effects creation tool with ASS and Lua.
A complete 2D Game Engine with built in drag and drop editor
Make games with Lego-like building blocks