I had this idea about a collaborative web application for many years. In short - the system embeds a small, simple visualization into web pages, next to the browser's scroll bar. Triangles are used to represent single users and their position on the current page (scroll position). Collectively, this allows you to see where people are congregating on a web page - perhaps next to a great deal, interesting news story, or funny video.
In 2008, Yahoo! HackU came to Carnegie Mellon's campus. The hack competition gave teams 24 hours to cobble something cool together. I suggested this idea to Brian and Julia (fellow rafters). They liked it, and so we threw it together in about 17 hours. Awesomely enough, we won first place, and a trip to New York City for Yahoo's 2009 OpenHack. There, with encouragement from Yahoo, we hacked on several new features. To add icing on the cake, Inhabited web picked up "Best UI".
The service is live and accepting beta testers. Check it out: