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.
An Atmospheric Scattering Demo, showing off a custom-written GLSL shader that creates sunrays shining over the mountains. To see it in action, go to Youtube.com.