iTunes Companion is a highly customizable, highly functional iTunes remote which searches for album cover art for your current iTunes track at amazon.com and downloads it to your hard drive. The next time you play that track, it finds the image on your hard drive. It searches for artwork in your iTunes Library before going to amazon.com. You can also choose to save the art to iTunes.
The Widget also searches for song lyrics. Lyrics are saved right into iTunes, so you can also see the lyrics on your iPod. It can also publish cover art and track info as an html page to a web server.
Source: Yahoo Widgets