Game engine supporting many 3D/2D formats and graphic effects
Orx: Portable Game Engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
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
TR Game Engine allows you to develop your game quickly and easily.
A truly extensible cross-platform game engine
Open source, moddable game engine for the "Polygon Love 2" game
Javascript adventure game engine