I've used various iTunes' controllers, usually in the form of Dashboard Widgets but not always, to do everything from finding artwork to sleep timers, but this little app called iTunesVolume beats all of them for both power and ease of use.
The name is deceptive, as this desktop-accessible volume slider packs a wealth of hidden features: it can access and display artwork, lyrics, and even commentary (I suspect it reads metadata files for the latter two features). You can also access your iTunes playlists, change volume, rewind, and fast-forward. Don't expect documentation of its features, though, as the developer is Japanese and his website offers only an inscrutable QuickTime movie that poses more questions than answers. Unless you read Japanese, you'll find no useful documentation of the features there. So here's a tip on how to get started: Click on the volume slider button and hold (while a song is playing), and that will open a menu which you can use to navigate all the hidden aspects of this great little app.
This app seems to have been translated to English by Sean Patrick O'Brien, so there may be a better explanation posted there, but a little rooting around in the app itself pays big dividends.
[robg adds: There are lots of iTunes controllers out there, and we've mentioned many of them in the past. This app is quite unique, so I thought it was worth covering as well. On the developer's site, you'll want to visit the Software page to download the app.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061228104932313