We recently stumbled upon this nice little app built on top of the Musicmobs and Last.fm APIs:
SimilarTunes works a lot like the “In Your Library” column of Mobster. Basically, it will take the related artist results from Musicmobs and build a playlist out of the recommendations that are available in your iTunes collection. This app is Mac only, but it’s Universal Binary so that’s a plus. Overall, it’s pretty slick.