cube 2 is a complete redesign of the cube engine developed by Wouter "Aardappel" van Oortmerssen.
This engine is really something special, I've seen some incredible things done with it. It has a simple premise but it is so much more. It allows for both things that are incredibly simple but fun and things that are amazingly complex.
Games that run on it perform great, at least I've never had any problems even on my older PC that could barely run most games Sauerbraten for example ran at very stable 45+ FPS.
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).