Well, that would be Pandora.com! (it’s free too)
This site basically asks you what your favourite song or artist is and then plays you random songs that are similar. The selections are based on other peoples recommendations in true “Web 2.0″ fashion (I’m beginning to hate that phrase!) and it’s pretty good for finding out about artists you haven’t heard of.
Apparently they play adverts every now and again, but I’ve been listening for a couple of hours and haven’t heard one yet so who knows what they are about…
From a web developers point of view, it’s also a sweet site: very AJAXy and with graphics that would make an Apple designer jealous.
Check it out
I'm a web developer based in the East Midlands, UK and if I keep up the current rate, I might have developed 3 million sites by the time I retire