Developer website: 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. Additune is free to use, free of ads, and will keep all your data secure.
Version v81: Leopard rewrite. Major changes to come.
Mac OS X 10.4 or later, iTunes 7 or later