Roblox - Reimagining the way people come together.
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
Limon Engine - Limon is a multi platform 3D game engine mainly focusing on first person games.
MOS - Lightweight game engine.