There’s a toolbar for Internet Explorer that has many of the same features as Firebug for Firefox.
Visit http://www.microsoft.com/downloads/details.aspx?familyid=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en to get a copy of the current beta.
To launch it, start IE7 and go to Tools -> Toolbars -> Explorer Bar -> IE DOM explorer.
You can inspect the DOM, the HTML elements, the CSS, and much more. It’s great to have this available!