libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Pygame - Pygame is a set of Python modules designed for writing games.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Unity - The multiplatform game creation tools for everyone.
MOS - Lightweight game engine.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.