Just recently on Stylish Scripting, Simon Willison introduced the possibilities presented by the XMLHttpRequest extension to JavaScript:

XMLHttpRequest is one of modern DHTML’s best kept secrets. If you haven’t encountered it before, it’s a method of making an HTTP call back to the hosting web server without refreshing the whole page – a kind of remote scripting on steroids. Originally a Microsoft extension, it’s been adapted by both the Mozilla browser family and (as of version 1.2) Safari.

It’s my understanding that Google uses XMLHttpRequest extensively in Gmail to help make it so fast and desktop application-like.