libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
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.
Unity - The multiplatform game creation tools for everyone.
MOS - Lightweight game engine.
Project Anarchy - Project Anarchy is a complete end to end game engine and state-of-the-art toolset for mobile.