http://www.activeworlds.com
Have you guys tried this? I claimed some land as a tourist and built a little house complete with bed and a couple of pictures on the wall using a tutorial.
Tourists' buildings are not permanent, so it will probably be demolished soon.
I guess the place boasts over 100 thousand registered users. Pretty amazing. I didn't see that many in there though. But there was a lot of architecture and many many worlds to go to.