I am the proud posessor of 45GB of music, none of which is downloaded. However, because I digitized many of these files on my PC years ago, most of my colection does not have album art. I found a simple fix for this problem.
First, I downloaded and installed the Sing that iTune! Dashboard widget. Activate the widget, and click the "i" button to flip it over. Check the Download Artwork button, and change the pop-up to 'into iTunes.' Then I downloaded a program that allows users to move widgets from the Dashboard to the desktop. I then moved Sing that iTune to the desktop.
Next, I configured it to save album art to iTunes, which also applies the artwork to all the songs from any given album. Finally, I set iTunes to shuffle.
Now, as iTunes shuffles through my music, Sing That iTune! goes through and applies all the album art. It's an easy way to get a lot of work done in just a few easy steps.
[robg adds: This hint explains how to detach widgets from the Dashboard without using any third party software. It may also be worth noting this hint, which claims that Sing That iTune! occasionally moved lyrics into the wrong song -- I don't know if this has been corrected in the current version. Finally, there are other methods of getting artwork into iTunes.
Also, if you missed out on getting PearLyrics while it was available, Sing That iTune! looks to be a good replacement app for getting the lyrics to songs you legally own. It's probably still available because it's not developed in the US, and as such, is somewhat safer from the reach of the RIAA.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060514151438270