If you reference an ASP.NET page that contains a scriptmanager in your js file like below you get intellisense for the entire Microsoft AJAX Library plus any additional scripts ref’d by the scriptmanager.  The easiest way to get the /// <reference/> entry in your js file is to drag n drop your .aspx page from solution explorer into  the contents of your js file.  VS magically adds the refJ

Since I tend to use stand-alone .js files I can just add this.

/// <reference name="MicrosoftAjax.debug.js" />

to my consuming page and intellisense is wired to my JavaScript code.

Thanks to Marc Schweigert for the reminder !