Reality Factory - Reality Factory is a free suite of game development tools for building 3D games.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Project Anarchy - Project Anarchy is a complete end to end game engine and state-of-the-art toolset for mobile.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Wicked Engine - C++ game engine focusing on modern rendering techniques.
MOS - Lightweight game engine.