XmlHttpRequest popped up on my radar a few months ago when Matt covered it. Back then, everyone and their brother was talking about Google Suggest. Haven’t found time to play with it yet, but I like the idea of asynchronous url requests. There’s lots of power there, not least the ability to make pull down lists dynamic without shipping everything to the browser or submitting a form via javascript.
I found a great tutorial on XmlHttpRequest by Drew McLellan, who also has a interesting blog. Browser based apps are getting better and better UIs, as Rands notices.
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Dion and Ben G. have started a blog just on this topic, http://www.ajaxian.com/