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:
- 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.
See full details: http://www.asp.net/ajax/