[Pxtl]

There used to be an IM application in the '90s called Odigo that was tied to your browser window - it would create a sort of ad-hoc chat-room based on your browsing. The problem was that it didn't intelligently parse URL paramaters so you had only "this exact URL that nobody else sees because of some hyper-specific stuff in the URL" and "everybody on the same domain" which was worthless for, say, Geocities.

I'm sure somebody is working on a modern web-ish version. You'd want to properly hack it for popular sites to parse URL parameters so that it would create rooms at the correct granularity.