libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
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.
Godot Engine - Feature-packed 2D and 3D open source game engine.