Windows only: Music application TuneUp scans your iTunes library to fill in and clean up your music's metadata, including album art. After you install it, just point it at songs in your library you want to clean up; TuneUp fingerprints and analyzes them and then provides a diagnostic overview of your missing or incorrect metadata. You can then verify and clean up all your metadata with the stroke of a button. In theory it's very similar to previously mentioned MusicBrainz, but it's an altogether fresh take with a few more features.
For example, similar to the previously mentioned iConcertCal, TuneUp lists upcoming concerts for artists in your library; it also lets you explore your music through a web digest of YouTube videos, articles, and merchandise related to bands you like. I just finished testing TuneUp on around 40 entirely unlabeled or poorly labelled tracks, and the results have been impressive. Not only does the app update the metadata with accurate, consistent information, but it does it all live, updating your library on the fly. TuneUp is ad-supported and free to use, but you're limited to 500 cleans and 50 cover art look-ups per month. A premium version without any limitations is also available for $US12 annually or a lifetime price of $US20. TuneUp is Windows only, with a Mac version planned for later this year.