Unity - The multiplatform game creation tools for everyone.
Lumix Engine - Open source 3D game designer and engine written in C++.
Struckd - 3D Game Creator - Struckd is the next-generation gaming platform for user created content where people of all ages...
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Reality Factory - Reality Factory is a free suite of game development tools for building 3D games.
MOS - Lightweight game engine.