The reason I started this post is, I think the best thing that have happened in last two years on the Technology side, as far as I am concerned, is Pandora. It's really awesome. Those who haven't yet found Pandora: it's a Music Genome Project. The site says:
Together we set out to capture the essence of music at the most fundamental level. We ended up assembling literally hundreds of musical attributes or "genes" into a very large Music Genome. Taken together these genes capture the unique and magical musical identity of a song - everything from melody, harmony and rhythm, to instrumentation, orchestration, arrangement, lyrics, and of course the rich world of singing and vocal harmony. It's not about what a band looks like, or what genre they supposedly belong to, or about who buys their records - it's about what each individual song sounds like.
So the idea is that you start with one of your favorite songs. Then based on the musical attributes of the song, Pandora will select other songs. For each song, you can tell Pandora whether you like this song or not, thereby 'training' Pandora to your taste. You can create 'Radio Stations' based upon this song. A 'Radio Station' can be created by Artist or by Genre too. There is a cool feature called Quick Mix, which is basically a random play of songs from Station you have selected.
It's really easier to experience and experimet with Pandora than explain how it works. You need a registration for creating Stations. But registration is free and require very little information. However, there is one caveat. The service is only for North American audience. I understand that is due to some licensing issue. If you are in North Americal (or have an North American IP address ... wink wink), go there and have some wonderful musical experience.