I happen to think last.fm is one of the best social sites on the 'net. I love the way it stays out of the way - requires very little interaction from me and it gives me recommendations in return: artists, albums and even concerts.
Recently, my favourite "tool" for last.fm, iScrobbler,released version 2.0. There are lots of new features, but the coolest I have to say is:
Full Radio support (played through iTunes).
What that means is you can use last.fm radio from iTunes (so, usual iTunes behaviour still applies). As someone who is frequently away from his entire music collection... I love that.