[jacquesm]

Once upon a time I built a whole service around that concept. It basically used what we today call cross site scripting to frame all websites you visit in the browser and then to use the frame to populate it with everybody else visiting the web that way. So you can suddenly see the crowd that is visiting the websites that you are visiting. And sometimes you'd notice the same people in more than one place. Avatars would 'drop' down on the webpage and you'd be able to chat with others by moving your avatar close to theirs on the screen and then to type a message, which would result in a popup text balloon over your character on their screen. It worked pretty well except for the largest sites where it would get too crowded, for those I eventually limited the radius based on the bitmask of the IP address until it was small enough to handle.

Cross site scripting protection in the browsers put an end to it but it was a fun project.