[How Do I:] Use the ASP.NET AJAX DropDown Control?
by Joe Stagner
With the DropDown control extender from the ASP.NET AJAX Control Toolkit, you can present the user with a drop-down list of options that trigger a partial-page update elsewhere on the Web page. The option selected by the user is passed to the server, allowing your server-side code to process the selection before completing the partial-page update.