Grooveshark is effing dopeass! I cannot endorse this site enough. Its basically an online jukebox - use it at parties when you want a nice mix of shit to listen to. I listen to it at work as another way to stick it to the man... (I also steal office supplies). You can pay $3 a month (or $30 a year) to become a VIP user (includes some bonus perks) but the free version serves me well.
Some other good spots:
Pandora.com (if you dont know about pandora you better ask somebody quick)
Mixunit.com (great for finding hip hop mixtapes)