Second Life is basically like one of the Sims games only you're not the only one playing in this world. Over a million other people are too.
From the site itself:
Second Life is a 3-D virtual world entirely built and owned by its residents. Since opening to the public in 2003, it has grown explosively and today is inhabited by a total of 1,243,436 people from around the globe.
This looks so really cool and absorbing and I did spend about an hour exploring the other day, but I came to the sad conclusion that I don't have nearly enough time these days to spend any significant time on this site. And you definitely need time to make this work well for you.
This is kind of a strange recommendation, right? Do as I say, not as I do. However, if you're into these kinds of games and you make time in your life for them already and as mentioned above you are among the microcosm of people who haven't yet heard of the site, all I'm saying is it might be worth your while to check this out. At the very least you will now know what the heck people are talking about.