I checked out the GTAIV theme based on this thread and its definitely the best laid out one I've seen. Still a little too busy looking for day to day use for me though.
Well now that I've seen the Transformers Movie I'm switching between the movie poster with Optimus/Megatron faces, Optimus vs. Bonecrusher and this wallpaper from the GM page:
Currently the Bungie Day free theme, but just because I wanted to see what it looked like and didn't bother to change back. It was this custom one before: