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.
Blender Game Engine is great. Why do i say this?
It has a good combination of features / against usability - anyone can start making a game. The built in modelling environment has proven to be incredibly handy at times!
The python API is strong, and easy to use :)