you put in the name of a song or an artist you like, and it'll play other songs that 'have the same musical qualities' or somehting like that. totally free, and you can give songs a thumbs-up or thumbs-down if you like em or not! you can have as many stations as you like for different types of music.
you have to register with a US postcode so if like me you're not in the US you'll have to find one (I used the Pentagon's!

it's great....having been a radio 2 fan for years i haven't even listened for a couple of months, as i now always have pandora on at work instead!