Coverflow

If you use iTunes for music on your Mac, then you have GOT to check out coverflow. It is FREE (for now) and takes you back to that wonderful old feeling of sifting through your LPs

check it out at http://www.steelskies.com/

It is worth bearing in mind that although coverflow looks for album art online, it seems to work MUCH better if you have already embedded the cover art into iTunes.
There is a free tool for doing this called Fetch Art...

You can get that from http://www.macupdate.com/info.php/id/12083

For people like me who have a lot of world music it is worth bearing in mind that if your iTunes library contains unusual characters (such as arabic or Greek or accented characters that are not part of the European lexicon) the program will fall over. This isn't a problem, you just need to note where it has stopped, force quit, rectify the problem in iTunes and relaunch Fetch Art. - or alternatively clean up your iTunes library in advance.