GrowlTunes is a very small app that adds notifications to iTunes. The app obviously uses Growl to deliver these notifications. When you are playing music on iTunes and a new song starts playing, you will get a notification that shows the name of the song, the album it comes from, the artist, and if you have artwork available, it will also display it. By default, Growl shows notifications in a small, transparent square, but you can change this on the Growl preferences window.
GrowlTunes comes with the main Growl download. Once you mount the DMG file, you will see that along with the Growl app there are a few folders. Under "Extras", you will find GrowlTunes, you can simply double-click it to run it or move it to the Applications folder if you want to run it the next time you boot your system. While running, GrowlTunes displays a menu in your menu bar. From there, you can see what song is playing and even access the iTunes rating system to rate it from 1 star to 5 starts.
All in all, this is a small, light app that can give you notifications when you are not looking at the iTunes window. It worked great for me and it was easy to install.