cube 2 is a complete redesign of the cube engine developed by Wouter "Aardappel" van Oortmerssen.
A good engine for to work with a fast progress curve, the only drama here is you must implement your own stuff by Source Code natively in C++ and if you want to modify something you need to understand the code by yourself (without any API or something who helps you)
One of a kind.
Built-in map editor, mesh exporter (the built-in editor doubles as a 3D modeler in itself), fancy shaders, scriptable game content, and the source code is even provided. What more could you ask for in a free engine.
This has to be the most inspirational game engine out there. In my opinion this beats several commercial game-engines by a long stretch.
Worth checking out, especially if you are into content creation (maps, models, etc).