Lumix Engine - Open source 3D game designer and engine written in C++.
Unity - The multiplatform game creation tools for everyone.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Godot Engine - Feature-packed 2D and 3D open source game engine.
MOS - Lightweight game engine.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.