"It's not free, but TuneUp is a really good product. Did wonders on my library, which had a ton of music that I took from podcasts (song of the day stuff). Made it pretty simple to get the right album and meta-data as well as the album art.
TuneUp is able to intelligently fix your music library using cutting-edge acoustic fingerprinting technology, which evaluates the song's digital DNA and references it against Gracenote's MusicID™, the world's largest database of music metadata."
"I've been on the hunt myself and haven't found anything as good as mp3tag for windows. So I've got that in virtualbox and playonmac (foss all the way).
Picard is nice in that you can add any fields you want (I often edit TCOP/WCOP/WOAF when releasing tracks under the creative commons) but I can't figure out how to change album art with it.
For a scriptable solution I've crawled back to the command line and have been using the free eyed3 which can be installed via macports
id3 editor is the fastest launching solution I've found but it costs money and doesn't have access to any custom fields."
