I use Advanced MP3 Catalog Pro, as suggested by someone on irc a while back. It's not fully automated, but I wouldn't want it to be. The automated ones I've tried would grab the wrong album info.
I stick all my incoming (newly downloaded) albums in a folder called incoming (subfolder for each album), then use AMCP to scan the album folders. It'll show a list of possible matches from FreeDB and Amazon. I select the correct album and click Apply and it fills in all the ID3 info on the mp3. It also has rename functions, which is nice for the albums that you download that have retarded filenames like "01_-_album.name_-_album.artist_-_song.name.here...[blahblah].mp3"