Xash3D FWGS engine
Open source library for creative expression on the web, desktop, etc.
Orx: Portable Game Engine
GemRB is a portable open-source implementation of the Infinity Engine
Cross-platform 2D game engine (+ full OpenGL, +full OpenGL ES )
Game engine supporting many 3D/2D formats and graphic effects
Portable game engine with main focus on point-and-click adventures
online 4X browser game engine