The Blender Game Engine is a component of Blender, a free and open-source comprehensive 3D production suite, used for making real-time interactive content. The game engine was written from scratch in C++ as a mostly independent component, and includes support for features such as Python scripting and OpenAL 3D sound.
This is a fantastic open source 3D modeler, animator, game engine....it does it all. Throw Python and a great Community into the mix and the sky's the limit. I love Blender.