A number of you are going to say I'm pretty slow on the take, as Pandora has been around for a while.
Customized listening- you build your own radio stations. Choose an artist you like- say Dean Martin, and then the station will choose artists to add to your radio station LIKE Dean Martin. If you don't like a certain song- give it a thumbs down and the station will never play that song ever again.
Love a particular tune? Type in the song name and Pandora will build a radio station around it.
Best part- Pandora is FREE.