XHTML Chat is barely more then a single html page + CSS + javascript. The server side bit is as small as it can be and could be accomplished with almost any available technology (down to php + textfile). This looks very well revolutionise the way the web looks and feels today.
Apart from this being a chat you can use XHTML Chat as:

• An advanced comment function for blogs offering immediate communication between users.
• An advanced shoutbox.
• A live single/multi-user publishing system.
• A XHTML Broadcasting tool for live coverage of events.

All of these could degrade gracefully if javascript is not available. XHTML Chat (including XHTML, CSS, Javascript, PHP, all images) ,is licensed under a Creative Commons License as well.

Requirements: -
Demo: http://chat.plasticshore.com/
License: Creative Commons License