As said in their web, Linden Labs has dual licensed their Second Life client both proprietary and GPL. But that isn’t everything: a new Linux client has been developed, so we all can enjoy the experience, and there are even people talking about using Mono as a scripting engine.
I have tried Second Life, and the least I can tell you is that it’s simply amazing. You can create objects and animate them writing scripts, perform all kind of actions (riding bikes, driving cars, talking, listening music, watching videos (ok, only through QuickTime in Windows), buying and selling objects, meeting people… in summary living a life. You should give it a try.
As a sample, I show you this picture, where my avatar is editing her motorcycle and seeing the source code of the script that drives it (click to enlarge):