I have a well organized mp3 collection and stumbled upon MP3 Album Art Tool (http://str8dog.com/default.aspx), a tool for automatically retrieving album cover art for each album you have. Together with this winamp plugin: http://www.winamp.com/plugins/details.php?id=138565, you get nice cover art while listening to your music.
Of course in practice it doesn’t find everything but
I consider a 60% score on my collection quite excellent
since it includes a lot of stuff that is pretty hard to
find anyway. The tool saves a nice folder.jpg in each
album folder which means that windows explorer can be
tricked into using the album art as a folder icon 
.