Voxel-based shooter with awesome ingame level editor. I only ever used it as a sandbox for creating worlds in multiplayer and had a big fun with it. The voxels are small enough to create enough detail. Only the GUI isn't so good.
I'm using this for simpler programs - there's all you need for basic games, but some advanced features, such as per-pixel lighting, are missing. Also some features are not supported with OpenGL backend. For advanced graphics use OSG or Ogre3D.
I finished the game and enjoyed it very much. It's addictive and fun. The assets can't be compared to AAA games, but are good enough for a free game and very cute. There's even basic storyline, which I'd like to see expanded a bit in the future.
I'd give it 6 stars. Blender could fight Linux kernel for the best FOSS software ever. Not only is it professional quality advanced 3D editor with multiple renderers and a game engine, it is also a video editor. It is scriptable in Python. The only downside is the unusual GUI, but it's justified and can be learned.