ASP.NET 2.0 AJAX Extensions 1.0 framework was released some days ago. You can use it for developing and running AJAX-style applications. Download the framework and the documentation or just read more about ASP.NET AJAX.
ASP.NET AJAX Control Toolkit was also released along with the framework. It is a shared-source community project consisting of samples and components that make it easier to work with AJAX-enabled controls and extenders. Go to preview the demo or download the toolkit.ajax, asp.net, control toolkit, extensions framework