iTunesVolume (download link below) is an awesome little program that I found a few days ago via macosxhints. It has a number of features - the ability to start and stop iTunes, change the volume, rate the current track, and display the current track's info and lyrics - all conveniently accessed from the desktop! My favorite feature, however, is that it displays the current track's album artwork on the desktop (fully resizable). I love being able to see the album artwork of the track I'm playing, and I think that the desktop is a perfect place to put it, but few programs offer this option.
I've been using iTunesVolume for a few days now and it works well, always changing the artwork within seconds of changing tracks on iTunes. The display preferences are numerous - the three components can made transparent and not all of them need to be shown. I only have the volume controller and the artwork running, as seen in the screen capture below:
One caveat: activity monitor shows that this program seems to utilize the CPU more than it should. I didn't notice any significant speed issues while I was using it, but bear that in mind if your Mac seems slower after installing it.
P.S. Jokes relating to Coldplay and The 40-Year Old Virgin will not be appreciated ;)