A particularly overlooked and, for its simplicity, useful feature of ASP.NET AJAX is the WebMethod, which is a server-side method that can be called by client script. The .NET compiler automatically adds client-side methods that take care of the AJAX request, JSON encoding and, to a degree, data type conversion. Sure, you COULD do this using a web service and XmlHttpRequest - but this is a quick-and-easy way of AJAX-enabling standard web form pages.