Last.fm is great, but I think I've discovered a new way of socially exploring exciting new music. It's called "public transport".
Step 1: Get on a bus. Student-loaded buses work particularly well.
Step 2: Cast a glance around you to identify positions of people wearing earheadphones.
Step 3: Wait until you hear something you might like the sound of.
Step 4: Ask person what they're listening to.
There are, of course, some drawbacks to this approach. Firstly, if you like quiet music then you might have a hard time finding stuff. Secondly, the "preview" option isn't much cop and often tends to be quite tinny. Thirdly, you might get odd looks. But then this is no social music service for JESSIES, y'hear? (Unless your name actually is Jessie.)
The other upside is that you can ask people who's music you don't like the sound of - chances are, they'll be so embarassed to tell you (c'mon, nobody really likes Coldplay...) that they'll turn the music right down to avoid being asked by anyone else. Cashback!
What are you waiting for? Get out there and start "podinterrupting".