A few weeks ago I discovered pandora.com, a site like last.fm which is probably somewhat more well-known.
You enter an artist or a song, and based upon that the site creates a radio station with similarly sounding music. They have some bizarre but at the same time cool licensing going on which doesn’t allow you to listen to the exact song you want, can’t skip back in the list, and only skip ahead x times per hour, but I’ve created a few stations (Tiga, 2 Many DJ’s, Tiesto and Vitalic), they have a pretty varying bunch of tunes… Songs you don’t like you can vote down and they probably won’t be played again. Songs you like can be voted up and will return. Been using it at work for background music for quite a while now.
Oh, by the way.. you have to live in US to be allowed to use it. Hey Beverly Hills (*cough* 90210), here I come 
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