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.
Lumix Engine - Open source 3D game designer and engine written in C++.
Project Anarchy - Project Anarchy is a complete end to end game engine and state-of-the-art toolset for mobile.
MOS - Lightweight game engine.
Wicked Engine - C++ game engine focusing on modern rendering techniques.