Lumix Engine - Open source 3D game designer and engine written in C++.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Sculptris - Sculptris: Enter a world of digital art without barriers.
MOS - Lightweight game engine.
ZBrush - ZBrush is a digital sculpting and painting software solution.