Wait, we've heard this before, haven't we? MyBlogLog (recently acquired by Yahoo) shows you who is viewing a Web page when you visit it. It's a really interesting app, but can be quite creepy.
A similar idea: Me.dium, which is launching its new "semi-public beta" at Demo 07 (which means it's an open beta, but new signups could be shut down at any time). Like MyBlogLog, Me.dium shows you who else is on the site you're on. It also shows you where your friends are hanging out online. And it shows you where people go to from the site you are on at any moment. You can chat with people who are on the site you're on, too.
Me.dium is a browser plug-in, so it works on any site.
Me.dium has privacy controls: it's easy to make yourself invisible to other users, and by default it doesn't show your presence on banking sites or on secure connections.
The service has real potential to make browsing--right now a private experience--a social experience. Unfortunately, both how to take advantage of the concept of Me.dium. and the features on the service itself are non-obvious to the casual user. It's harder to get up to speed on the service than it should be. But Me.dium is doing something very interesting and powerful, and it's worth puzzling through the service to understand its potential.