ASP.NET AJAX is a free framework for quickly creating efficient and interactive Web applications that work across all popular browsers.

ASP.NET AJAX is built-into ASP.NET 3.5. It is also available as a separate download for ASP.NET 2.0.

With ASP.NET AJAX you can:

Create next-generation interfaces with reusable AJAX components.

Enhance existing pages using powerful AJAX controls with support for all modern browsers.
Access remote services and data from the browser without tons of complicated script.

Take advantage of the improved efficiency and ease of use in Visual Studio 2008, with its built-in support for ASP.NET AJAX, improved JavaScript support, and a new Web page designer interface.

http://www.asp.net/ajax/