If you're like most folks, your music library is a mess. Artist names are all over the place, song titles missing or misspelled. How many songs listed as "Track 1" do you have? I have seventeen.
Similar to TuneUp, MusicBrainz Picard is a user-generated database that identifies the songs in your library and adds the correct song title and artist name. It's a "user-maintained community music metadatabase." Unlike TuneUp, however, it cannot add album art or lyrics, which are protected under copyright. And, unlike TuneUp, it's free and run by a nonprofit group. While TuneUp relies on Gracenote, MusicBrainz is built from user input. Consider it a music Wiki that you can also add your own info. See a mistake? Log in and correct it.
Now, MusicBrainz isn't nearly as easy to use with iTunes as TuneUp because it requires a few more steps and file transfers, but hey, it's free, and you get to help make it better.