I'm sure alot of you know about this, but check out
Pandora.com. It's an extension of the music genome project and was made public last year.
Basically, it is an adaptive database of over 300,000 songs. To begin, you enter the name of a favourite song or artist. Pandora will then generate a playlist featuring songs and artist that are similar to your favourite. Either that match the song composition - genre, rythmn, beat count, speed - or that other users who listen to similar types of music have labelled positively. You can then add more bands / favourite songs as influences to create a totally diverse and constantly updated playlist of songs you will like!
You can have multiple stations or mash them all into one. Oh yeah, this is all legal by the way... duh... but some legalities are a problem. Such as you are not able to skip tracks more than ten times per hour, but you can ban songs you dont like from ever appearing on your playlist again and if you have to skip more than ten times an hour, you can change over to another station and then change back instantly.
It's a great way to discover new bands and of course, your 'stations' are saved and adapted overtime. As this project grows, it will become more accurate and even more impressive.
Another great thing about Pandora is - subject to approval - you get paid for advertising their website. I am recieving around $2,000 of your US dollars. LOL, jk, it just seemed like i was plugging them too much. It's good though, check it out.