Additune

Additune is the first major Mac only music recommendation app. It understands that the kind of music you want to listen to depends not only on what you like in general, but what you're in the mood for at the moment. Select one of your iTunes playlists in Additune and you'll get recommendations custom tailored to that playlist. You can choose to see recommendations for tracks you don't yet have or for ones that are already in your library. Additune is free to use, free of ads, and will keep all private info.

Requirements:

Mac OS X 10.4