Clutter works, though it doesn't work too well for me. It can't find artwork for most of my songs, and I have pretty mainstream songs. Clutter, however, is free. When you're playing a song, it'll automatically hunt down artwork. It also has this neat feature where you can put a picture of the album cover on your desktop, then click on the picture for quick access to songs from that album.
There's
Export Artwork as well, but this seems to have a bug where, if you have multiple albums from the same artist, it will only fetch ONE album for all of those songs. This is kind of annoying. It's shareware, so I don't know if that's just a way of annoying you so that you'll buy the software.