"ASP.NET AJAX is a free framework for quickly creating a new generation of more efficient and interactive Web experiences that work across all the most popular browsers.

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.
Continue using Visual Studio 2005 to take your ASP.NET 2.0 sites to the next level.
Access remote services and data from the browser without tons of complicated script.
Enjoy the benefits of a free framework with technical support provided by Microsoft." to find out more..

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