Surprise, surprise. Check out http://atlas.asp.net to try out a preview of Microsoft's AJAX framework. From the website

ASP.NET "Atlas" is a package of new Web development technologies that integrates an extensive set of client script libraries with the rich, server-based development platform of ASP.NET 2.0. "Atlas" enables you to develop Web applications that can update data on a Web page by making direct calls to a Web server — without needing to round trip the page. With "Atlas", you can take advantage of the best of ASP.NET and server-side code while doing much of the work in the browser, enabling a richer user experience.

ASP.NET "Atlas" includes:

• Client script libraries that provide a complete solution for creating client-based Web applications. The client script libraries support object-oriented development, cross-browser compatibility, asynchronous calls to Web services, and behaviors and components for creating a full-featured UI.
• Web server controls that provide a declarative way to emit markup and client script for "Atlas" features.
• Web services, such as ASP.NET profiles, that can add useful server-side features to an "Atlas" application.

It's great to see this getting out to developers so quickly. Kudos to Scott Isaacs, Walter Hsueh and the rest of the folks at MSN who worked with the ASP.NET team on this project.