Very Dynamic Web Interfaces, another nice introduction to XMLHttpRequest.

Also XSLT in JavaScript for Gecko. (Thanks Jim.)

Neither of these technologies have an official implementation standard in browsers, but their presence and common implementation in both IE and Mozilla/Firefox/Netscape are bound to make them de facto. The crazy-domain-insane stuff Google is doing (you've seen Google Maps by now, I hope?) has greatly amplified their popularity, and they're sure to be an essential part of every web application developer's toolkit by this time next year.