I'm using the free version of Bumptop. It's pretty cool, though it does seem to have wiped my iTunes library for some reason...
I just got it so the desktop isn't actually that pretty yet, but it is fun.
As far as I see there's no good reason to pay for the full version, it doesn't add much.
EDIT: changed background and other things to make it prettier looking. Doesn't really deserve another post.