Microsoft just released the new version of ASP.NET AJAX as Beta, there are going out of the CTP cycles.
With ASP.NET AJAX, developers can quickly create pages with rich, responsive UI and more efficient client-server communication by simply adding a few server controls to their pages. This new Web development technology from Microsoft integrates cross-browser client script libraries with the ASP.NET 2.0 development framework. ASP.NET AJAX provides developers building client-based Web experiences with a familiar development process and programming model that they already know from using server-side ASP.NET development. Because ASP.NET AJAX is integrated with ASP.NET, developers have full access to the built-in ASP.NET 2.0 application services and the entire .NET Framework.