KlayGE is an open source, cross-platform game engine with plugin-based architecture. It provides a framework to make game development, testing, porting as simple as possible.
pyGLFW is a set of python bindings to the successful cross-platform GLFW. It is intended as a powerful and lightweight alternative to pygame for python OpenGL application development.
Spineless is a generic cross-platform 3D game engine implemented in Python. Focus is on flexibility, clean design and ease of use, not pure speed or bleeding edge graphics.
MayaVi is a free, cross platform, easy to use scientific data visualizer. It provides a GUI to ease the visualization process, is written in Python and uses the Visualization Toolkit (VTK) for the graphics.
Xgdk is a Cross-Platform Game Development toolKit. Its purpose is to provide a set of easy-to-use and extendable libraries for rapid development of 3d interactive applications such as videogames, demos and prototypes.