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.

Watch ASP.NET AJAX Development in Action

Watch these videos to see what you can do with ASP.NET AJAX in your own applications!

ASP.NET AJAX: A demonstration of ASP.NET AJAX

See how easy it is to build from scratch a "To Do List" management application using ASP.NET 2.0, and then enhance the user experience using the ASP.NET AJAX Extensions.
ASP.NET AJAX Support in Visual Studio 2008:

You will get a quick overview of the inclusion of ASP.NET AJAX into Visual Studio 2008 (Orcas) and .NET 3.5. This will be demonstrated by using Visual Studio 2008 and the building of web site application.
JavaScript Intellisense Support in Visual Studio 2008:

In this video I will highlight some of JavaScript Intellisense built into Visual Studio 2008. I will show ways of improving the Intellisense JavaScript feature by the way you write your JavaScript code.
JavaScript Debugging in Visual Studio 2008:

Using an existing web application, I will show you how to use Visual Studio 2008 to debug JavaScript in a web application.

» View more ASP.NET AJAX videos

ASP.NET Previews on Codeplex

The ASP.NET AJAX Previews on CodePlex provide new features that extend the ASP.NET AJAX framework and that might be included in future ASP.NET AJAX releases. These CodePlex releases are not considered production quality and are intended for use only by experienced developers.

For more information, see Welcome to ASP.NET AJAX 4.0 Preview 3.