scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
Cross-platform 2D and 3D game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
Egret is a brand new open mobile game and application engine
Visual Novel engine, editor and game development.