By accident I have seen that Heiner Wolf (he was a researcher at the University of Ulm when I was a student there) has a new company - http://portal.zweitgeist.com/. Back then in 1996 he worked on a European funded project called CoBrow - Cooperative Browsing - looking how to create a system that allows you to meet other people who are on the same web page as you are.
Since then I think that is a really exiting idea. But looking at his current work it is easy to imagine that this could really take of. It is extremely easy and looks like fun. You have your avatar (which you can customize) that can meet and interact with other avatars that are at the same web page as you are. It is not a lot what you can do - but that is the beauty!