Unity - The multiplatform game creation tools for everyone.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Godot Engine - Feature-packed 2D and 3D open source game engine.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
MOS - Lightweight game engine.