From Phil Windley - UI Design Is Different with XmlHttpRequest. There has been a ton of great stuff recently coming from XmlHttpRequest - it's hit the tipping point. I really enjoy seeing mature consideration of the possibilities - when Jakob Nielsen starts grumbling about it, you know it's arrived.
I'd love it if someone did a background story on who created that and got it added to IE4. When I first saw it four or five years ago, I said "This is the most important thing to happen to the browser since the browser." At DataChannel, a buddy quickly moved on it and implemented a Java version with the same interfaces in an applet we had that supported XML parsing and XSLT transformation - remember, this was before anyone had XML parsers in the browser.
I think XmlHttpRequest will go down as a major innovation on par with the browser itself.