I had the opportunity to attend Mix06 in Las Vegas a couple weeks ago. What a great conference! While it didn't dive in as deep as some other conferences, the overview of technologies, the networking, and the level of people that were there made this conference well worthwhile. My favorite session was the Microsoft.com and Myspace.com guys discussing their webfarm configurations. Msn.com (managed by the Microsoft.com team) and Myspace.com are two of the top 5 most popular websites at the moment.
For those that aren't familiar with it, AJAX allows a webpage to only update the part of a page that needs to be updated. The rest of the page doesn't do a round-trip (postback) to the server. For example, with maps.google.com you can drag your mouse around and watch the map appear as you move around. That was obviously not preloaded into your computer, but it will pull the images back from the server as you request them.
In March, on the first day of the MIX06 conference, Microsoft released a public version of 'Atlas', their framework to AJAX-enable a website. Another version was released in April and as of two days ago, April 13th, the CTP (community tech preview) of Atlas Control Toolkit was released which includes an impressive set of AJAX controls. Scott Guthrie says they are planning to have between 50 - 100 high-quality Atlas controls over the next few months: http://weblogs.asp.net/scottgu/archive/2006/04/13/442793.aspx
Take a minute and try the a demo of the Toolkit samples, you'll be impressed: http://atlas.asp.net/atlastoolkit/CascadingDropDown/CascadingDropDown.aspx.