Simpsons map for Quake III Arena - Boing Boing
As these game editors are so easy to use I'm surprised not to see a lot more experimental or artistic work taking place using them. I'm working on an installation using the Unreal 3 engine right now but haven't seen a great deal of other stuff.
I love this work by Alison Mealey (interview here) that uses Unreal Tournament processes to generate visual art.

Kurt Hentschläger has produced this work 'KARMA/cell' using the Unreal 3 engine.
