music recommendation sites

The three best music recommendation sites that I know of are:

http://www.liveplasma.com/- This is based on what CDs people buy together on Amazon.com. In other words, if people buy Madonna CDs along with Christina Aguilera CDs, then those names will appear together.

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http://pandora.com/This database is prepared by musicians and musical academics. This site will play music for you, ask you to rank or rate it, then plays recommendations.

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http://last.fm
Click on Music, and type in the name of an artist, such as Madonna:
http://www.last.fm/music/Madonna
Click on the "Similar" tab, and you will see a list of artists that people listen to most, who also listen to say, Madonna:
http://www.last.fm/music/Madonna/+similar

Enjoy!