I just ran across the Pandora music player… Wow.
The idea is to identify a song or artist that you like, and by using the Music Genome Project Pandora tries to identify similar songs and artists. You can guide it by saying you like a certain song or dislike it, you can prevent songs from being played for a month or so, etc.
The whole player is browser-based (it uses Flash), and has a lot of neat little features (like a progress bar underneath the album cover for the song, and cool little animations when you provide song feedback).
This is a free service that shows and displays ads, or you can subscribe to an ads-free version for $3-4 a month depending on the subscription plan.