In this video, Todd Miranda demonstrates how to add AJAX functionality to an existing ASP.NET page with minimal effort to increase performance and improve the overall user experience of your application.
http://msdn.microsoft.com/en-gb/vstudio/bb821246.aspx

This presentation recaps how to use the Ajax.ActionLink and Ajax.BeginForm Ajax Helpers to utilize ASP.NET AJAX in your ASP.NET MVC Web Applications.
http://www.tampadev.org/News/Details/MvcAjaxHelpersScreencast

This video demonstrates how to use the AJAX Control Toolkit to create a new custom AJAX Control Extender.

In this video you’ll learn how to set up your Development Environment for developing ASP.NET AJAX Applications if you are targeting ASP.NET 3.5.

This video demonstrates how to use the AJAX Control Toolkit to create a new custom AJAX Control Extender.