Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Pygame - Pygame is a set of Python modules designed for writing games.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Synfig Studio - Just another WordPress site
Unigine - Unigine Engine is a proprietary cross-platform middleware, developed by Unigine Corp.