A Google Chrome extension that allows people visiting the same URL start chatting anonymously. There is no account registration or sign in. You just install the extension and start talking to people immediately.
In this extension, there is a centralized web server which hosts all the URL-based conversations. The client is implemented with Comet model to allow the server to push the messages to clients. The source code of this extension could be checked out from Google Project Hosting (http://code.google.com/p/chat-everywhere).
Check to see how it runs by installing it from on Google Chrome Web Store