Most people know about Pandora. It's an online streaming radio interface that customizes the stations to your liking. You punch in some artists you like and it develops a play list of similar artists that you might enjoy. You can always move or remove artists from a particular playlist.
It's AWESOME - you don't have to download anything, just stream off the computer.
I found this interesting article, didn't know they had a paid service but it should pretty sweet. At $36 a year how could you really go wrong? Pandora One- Better than the Free Version