The program I use is called Clutter. It lists my iTunes library and browses through Amazon.com for album covers. Once it locates an album cover you make it copy to iTunes and then the artwork is part of the files. This will make each file bigger so make sure your artwork selection is a reasonable size.
Hmmm, I probably should have been clearer. I have ~1000 songs that already have album art embedded into the MP3 file. I'd like an app that allows me to browse through them, just like a picture book of sorts...
Flat6 needs something that can display album covers the way iPhoto displays photos. No such application exists but I guess that with Automator in OS X 10.4 you could create an Applescript that exports album covers into iPhoto.
Maybe the next iTunes could feature something like what Flat6 is asking for.