From developer Sam Rayor via one of my favorite websites Lifehacker comes a handy tool to help music collectors round out their iTunes collection. Discographer scans your library and alerts you of missing albums for artists. From Lifehacker:

All you need to do is upload your iTunes library XML file (located at /Users/username/Music/iTunes/iTunes Library.xml) and wait a few minutes while Discographer scans the file. When it’s done, you’ll get a list of all your artists so you can see which albums you’re missing. For completionists, it’s a pretty handy way to round out your collection. Plus, your library info is never uploaded, shared, or saved.

What's interesting is this uses the Spotify API to scan and search and never uploads your library. I will certainly be using this to round out my collection.