Pandora is my latest obsession. It really is simply amazing. It's basically an internet radio station that modifies it's playlist based on your tastes in music. And it's free!! Well, supported by ads anyways.
I think this truely is the future of music as network access becomes ubiquitous. I love my iTunes, but that is just a glorified CD collection. Yahoo! Music is a more intelligent CD collection (i.e. subscription based, lives on the network). But Pandora introduces INTELLIGENCE into the equation. Mind you I don't think Pandora is the pinnacle.. something like being able to listen to an artist when you want would be necessary too..
Still, Pandora is amazing. Even the paid version at US$36 is a bargain!