Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Godot Engine - Feature-packed 2D and 3D open source game engine.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Cyberix3D - Free online 3D Game Maker. Make your own 3D games online!
MOS - Lightweight game engine.
Unity - The multiplatform game creation tools for everyone.