Specere, the people who brought you Menuet, comes Art Collector. What's Art Collector? An app that downloads album art from Amazon for your tunes in iTunes. Sure, there are other apps that do this, but the UI on this one is a cut above the rest. Plus, for you Menuet users, it is fully integrated with Menuet.
Art Collector costs $7.95 alone, though it is included with Menuet for no extra cost (Menuet runs $12.95).