The March CTP of Visual Studio Orcas is out, and it includes IntelliSense support for JScript. See Jeff King's post about it.

That by itself is cool, but the ASP.NET AJAX support is awesome! It uses the xml documentation in the AJAX Library to give you extra information like parameter types/descriptions.

I've seen this before internally, but I haven't gotten a chance to play with it yet. I'm definitely looking forward to it!