One of my professors mentioned Second Life about 6 months ago so I tried it. It's weird and I ended up closing my account after about 2 weeks.
People can get really addicted to it but, to me, it just seemed like an enormous set of elaborately decorated chat rooms where you can fly around.
The only part I liked was creating my "look." (You can look like just about anything.) It was fun for about 15 minutes. I did some scavenger hunts too and won some fake money....yawn.
Apparently you can "buy" or make things and sell them and if you have enough money you can buy property and design it anyway you want. I don't know. It just didn't grab me. I spent most of my time literally crashing into people, apologizing and then saying, "I'm sorry, I don't speak Portugese."
ETA: I think it mostly appeals to people who are into RPGs.