libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Unity - The multiplatform game creation tools for everyone.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Struckd - 3D Game Creator - Struckd is the next-generation gaming platform for user created content where people of all ages...
MOS - Lightweight game engine.
Reality Factory - Reality Factory is a free suite of game development tools for building 3D games.