Beathound.com is a cool service that will send you recommendations on what music is missing from your collection.
it analyses your itunes xml file and emails you recommendations based on the artists and albums that you have in your collection.
if it finds new releases by artists that you already have then ti will tell you about them too!
its a really good idea, go check it out at beathound.com
once your library xml file has been uploaded beathound will analyze it and will present you with a list.
you can ignore an artist entirely ant it wont tell you about them again, or you can ignore an album and it wont tell you about it again!