Cyberix3D - Free online 3D Game Maker. Make your own 3D games online!
Pygame - Pygame is a set of Python modules designed for writing games.
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
Unigine - Unigine Engine is a proprietary cross-platform middleware, developed by Unigine Corp.
Unity - The multiplatform game creation tools for everyone.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games