Online custom radio - Pandora

Pandora generates custom online radio stations, playing music specifically tailored to your tastes. Basically, you tell it what artists or songs you like and it makes a radio station for you to listen to that will play songs kind of like the ones you entered.

It's the child of the Music Genome Project, which apparently has a crack team of "music analysts" who delve into the guts of each individual song and then compare it to other songs, with an eye for compatibility. From their About page:

"Each song in the Music Genome Project is analyzed using up to 400 distinct musical characteristics by a trained music analyst. These attributes capture not only the musical identity of a song, but also the many significant qualities that are relevant to understanding the musical preferences of listeners."

Interesting theory, but I'll have to try it out. I've been using LastFM for some time now, with fair results, so we'll see...