The Ajax application included in this page implements a legitimate, if not particularly useful or even usable, "proof of concept" with partial page updates based on server communication. It accepts a string, and then lets you click on one of a few buttons to see that string styled the way the button is styled, appending a link from the server. But it demonstrates one interesting feature:
How does it work?
Ajax partial page updates don't need to manipulate a monolithic page's DOM; the reason browser back buttons work in Gmail is an invisible, seamless use of iframes that create browser history. And not only can you do partial page updates via iframes without DOM manipulation, you can do it without client side scripting.
The source code to the server is available here, but it is simple, stateless, and doesn't really hold any secrets; it could be fairly well reconstructed simply by observing what is going on in the demo app above. The basic insight is that a webpage that talks to a server and makes partial updates can be made by the usual Ajax tools, but at least a basic proof of concept can be made with old HTML features like frames and iframes, links and targets, forms, and meta refresh.