RetroArch is a frontend for emulators, game engines and media players.
A dead simple 2D game engine for Go
A promising chess program
Game engine supporting many 3D/2D formats and graphic effects
A cross-platform interpreter for many point-and-click adventure games
Android, DOS, FirefoxOS, Linux, MacOS and Windows all in one..
Paintown is a 2d fighting game engine.
Cross platform Game Creation Software