I have a huge iTunes library, more than 25,000 songs and 140+ GB. I've acquired my music from all over the place -- ripped from CD, downloaded, shared from friends, etc. As a result, many of the album, artist, and track names are a total wreck. Tunes are mislabeled, some artist names include "The" and some don't (Beatles vs. The Beatles), and a slew of tunes are titled "track 01," etc. A few days ago though, my friend Gabe Adiv fixed almost all my metadata for me. His company, TuneUp Media, just launched a plug-in for iTunes that cleans up your library's metadata and grabs the missing album cover art. It takes an "audio fingerprint" of each track and then gets the appropriate data from Gracenote's Global Media Database. It'll also let you know if you're missing any tracks from a particular album so you can buy them from iTunes or, er, find them elsewhere.
Right now, the TuneUp Companion is only available for Windows. (Mac version is coming in the Fall.) I don't use Windows, so last week I gave my hard drive to Gabe to test the software on his PC. He said it took about 12 hours to process my whole collection (10 hours to clean, 2 hours to get the cover art.) It couldn't find the metadata for approximately 500 tracks, but that doesn't surprise me because I easily have that many live concert bootlegs that aren't in Gracenote's database. The company claims they're averaging a correct rate of 85 to 90 percent. A quick flip through my library makes me think it worked even better than that for the metadata and about that well for album art.
TuneUp Companion has several other features that I haven't personally seen in action. It grabs contextual content from various places online. For example, if you're listening to "Creep" by Radiohead, the "Now Playing" feature will check YouTube for live videos of the song and search for bio info and music news. The Concert feature looks for tour information and can be set to alert you if a band is coming to your town. Gabe told me they're planning to open up the "Now Playing" API so anyone can create their own contextual content features.
For me though, the Clean feature is the big selling point. TuneUp costs $12 a year or $20 for a lifetime of use. A free version is available that cleans 500 songs so you can get a sense before you pay if it will work as well for you as it did for me.
TuneUp Companion demo video
Geek.com's TuneUp review
Download.com's TuneUp review