You might overlook the JavaScript debugging utility that's already built in to Visual Studio 2005: the Script Explorer window.  The Script Explorer can take care of the usual debugging tasks like stepping through code, adding watches and evaluating variables. 

To use the debugger in Internet Explorer navigate to the browser Tools | Internet Options | Advanced tab and make sure that "Disable script debugging" is turned off. 

Run your web application in Visual Studio 2005.  Then select the menu option for Debug | Windows | Script Explorer.  Notice in the background the tags for telerik RadEditor controls...

The first thing you notice in the Script Explorer window is a series of JavaScript and resource files that are currently loaded.  Double click on the aspx file you're currently working with and you will see the evaluated HTML returned from the server.  The RadEditor control now shows as its computed HTML, CSS and JavaScript that will actually be functioning in the browser. 

You can also navigate up to the script for the page and set breakpoints and watches.  When the JavaScript executes and hits your breakpoint you get all the usual Visual Studio debugging capabilities for free.

Next blog I'll show the excellent "Firebug" debugging utility for Firefox.  Firebug doesn't stop at just JavaScript but works with the entire stack of AJAX related technologies (and has a high cool-factor).