This free iTunes Konfabulator widget is amazing. It works on both Mac and XP and ties directly into iTunes and does some amazing stuff. It links in with Amazon and Google, automatically looking up your iTunes songs and tracks info, and fills it back into iTunes as you play the songs.
You can download it here, for either mac or windows. If you're not familiar with Knofabulator, it's a JavaScript runtime engine for Windows and Mac OS X that lets you run little files called Widgets that can do pretty much whatever you want them to. Widgets can be alarm clocks, calculators, can tell you your WiFi signal strength, will fetch the latest stock quotes for your preferred symbols, and even give your current local weather. Before downloading the widget, you need to download the engine, all of which is free, and get it here.