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http://www.albumreminder.com - It's pretty simple. You upload your iTunes library and create email notifications or a RSS feed to notify you when your favorite artists release a new album.
http://www.bandsintown.com - It would be great if this website also allows you to upload your iTunes library, instead you need to manually enter in your favorites. What bandsintown does is help you discover who is playing in town in the upcoming months. It's also great for traveling when you want to check out whose playing this weekend. The band tag cloud they offer is pretty useful as it indicates which bands are popular.