Only a few months ago, I was exposed to XML/Http for the first time, while Jeff Prosise gleefully demonstrated the support for it in Whidbey with ClientCallbacks. XMLHttp is not new at all, but because it only works in Internet Explorer, it has definitely been overlooked. XMLHttp allows partial web page updates without refreshing the entire page. You can go out and get some updated data from the server for your page, but without the expense of a full postback. Apparently this is part of the magic behind how Outlook Web Access has worked so beautifully - but in Internet Exporer only.
Since February, the blogosphere has been abuzz with AJAX, which had been used by GoogleMaps and impressed the pants off of a lot of web developers. AJAX incorporates XMLHttp but is not limited to I.E.
Now it is "hold the presses" at Microsoft who is planning to roll out ATLAS, explained here by Scott Guthrie, which will work on top of ASP.NET 2.0. The teaser is that we can see it first at PDC.