I've found quite a few cover only scripts, but their are two search scripts currently available that I would love to see a cover only script compiled from. I would do it myself, but I wouldn't have the slightest clue where to start.
I would love to see Discogs & MusicBrainz (w/ ID) as a cover only. I haven't tried the MusicBrainz yet to see if it actually has covers, but the script searchs for them. All my albums were tagged using Picard, but without any covers. Now I'm going through and adding covers to my 80gb music collection and since all my albums are tagged by picard a musicbrainz cover search should help ease my pain