Fetch Art for iTunes Beta 1.05 is a script that fetches album art for one or more MP3s using Amazon’s XML interface, and then inserts the art into the MP3s’ ID3 tags so that it can be displayed by iTunes. The beta release adds a new graphical interface, preview of images before copying to iTunes, more accurate searches, and a new installer. Fetch Art for iTunes is free for Mac OS X with the BSD subsystem installed, and iTunes 4.
Fetch Art for iTunes is controlled through the iTunes Script menu. In iTunes, select one or more songs, or select a playlist, then choose “Fetch Art” from the Script menu. The Fetch Art application will start, and attempt to download art for the songs you chose. After art downloading has finished, select any of the songs to see what art was found. If you are happy with the art that was found, hit Return to copy the art to iTunes. You can also copy art to iTunes for every song in the list by choosing “Fetch Art” > “Copy All Art to iTunes,” or by hitting command-T.
More information here.