Last.fm is cool.
Pandora is EASY.
If you are looking for an online community to sink, sink, sink your sharp, astute, teeth into then go with Last.fm
If you are at a friends house and you are jonesing for M.I.A. and your friend doesn't have the CD and your iPod is in the toilet (or you don't have one) then go to Pandora.com and start a station by typing in one name of one band and then have music bustin' out of da computer speakers in no time. And they play LOTS of whomever you type in. And it's easy.