A friend from work (Jeff) turned me on to Pandora, a website that streams music through "stations" based on an artist or song that you define. The definition is the seed that Pandora uses to match other songs through the Music Genome Project and plays them for you. By providing feedback, (in the form of thumbs up or thumbs down), Pandora changes the suggestions for your station. It will even tell you the parameters it put into suggesting the music. For instance, I started a Sting station, and it suggested Death Cab for Cutie because "it features mellow rock instrumentation and a subtle use of vocal harmony." It turns out, I really do like Adult Contemporary.

Since the music that it plays is new to you yet, strangely familiar it's quite good a being a front end for purchasing music (which you can do right from the image of the album cover).