oh my goodness! Pandora is going to be my favourite website for a long while!
Just type in an artist or song that you like, they'll search for it and start playing a whole bunch of songs that would be similar in style to your search criteria.
All for free!
How cool is that?
[Thanks to Seth Godin for the link.]
Update: D'oh! Seems like you have to register to actually listen to more than 4 tracks and have gotta be an American citizen to qualify...sigh...the idea was good though. Now for an Australian version...