For more than 2 years I have been using Yahoo Radio every now and then, specially when in the office with my headphone. The nice thing about Yahoo Radio is that it plays songs that are within your taste since you have to rate down and up the songs you listen. The down side of Yahoo Radio is the audio adverts that have been more and more numerous (and not entertaining at all while working!)
Now a collegue gave me this link : "Pandora", it's the same principle as Yahoo Music but this time you can create more than one radio (Yahoo only allows my own radio) like one per genre, then add a couple of albums or artist and it wil do the rest.
For instance look at the way it defines the songs "like " Fisherspooner (the band) : "Based on what you've told us so far, we're playing this track because it features mellow rock instrumentation, electronica influences and a subtle use of vocal harmony" , now I know why I like it then !
Pandora Internet Radio - Find New Music, Listen to Free Web Radio