It was called OLark if my memory hasn't failed me. From what I remember it was semi-popular for a while but ultimately died a death of user scarcity as only the most popular websites ever had >1 active user with the program installed. Let me try find the link....
[edit] http://news.ycombinator.com/item?id=801242
Sometime in the 90s there existed a Windows application that added a chat overlay to any webpage. Users had small rectangular icons and could move around the webpage. You could also create a "bus" and drive people around the web.
If I get time (have to concentrate mostly on job-hunting for now) I was thinking of doing a "auto-post-to-twitter" to try and pull people outside the conversation in.
