Second Life is like a sim kind of "world", if you will. It's not a networking thing, per se, but like playing the Sims with people around the world. People buy imaginary land, islands, nightclubs, etc...with REAL money. Apparently, there is an assload of money to be made off of this monster, people should really look into it. I, myself, haven't really gotten addicted to it like most, I just don't have the time.
I think the site is
www.secondlife.com