I launched this online service today that suggests songs to you that you wouldn't normally have or listen to, but that are still good. It's kind of like a reversed “iTunes Genius”… actually, maybe more of an “iTunes Librarian” that can help you find interesting books to read by just showing up to the library with a list of what you already have… except, for music. Hah.
The site works for just about anyone with enough indexable information in their iTunes Library (whether it be star ratings or even just “play counts,” as well as other factors like quantity of songs by that particular artist, etc.) — Give it a try, and grab some new tracks from those artists that you already know and love!