If you want to prefix your code to use the AJAX Control Toolkit, add in the following item to your web.config.
<system.web>
<pages>
<controls>
<add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
</controls>
</pages>
I think it helps out with the Intellisense in Visual Studio .NET 2005
FROM:Wallace B. McClure