i'm shocked to see second life get mentioned here.
for those who don't know SL is a virtual reality platform, some call it a game, but it's really a platform in which you can create almost anything. role playing and building stuff are two of the things it's used for. in it's hey day about 10 years back it was touted as the 3d internet. which it is in a way.
Open Simulator is an open source version of second life.