I'm "playing" the latest svn checkout of
Cube 2: Sauerbraten (a new release is due soon). The Quake 1-style gameplay is irritatingly simplistic, but the engine is impressive. In-game editing, an octree-based world structure, and it's open source.
An MMORPG based on this engine might be really cool. There was an effort to create a single player RPG, but it appears to be dead.