I was always inspiring the level editor in Cube 2: Sauerbraten (http://sauerbraten.org). This seems like an alternative to BSP approach which was more popular at the time. But I think this game's octree-based solution has more potential than just the render and raycast logic implemented in the engine. You could potentially make the entire level fully destructible and keep it optimized at the same time. The level building was also fun when I was teenager. No game allowed to edit levels with other players connected to the server and that was working in 2004!