IE Developer Toolbar

On its way to try to catch up with Firefox, IE now features a Developer Toolbar that gives immediate access to a set of fairly cool tools to debug your Web pages, including a DOM explorer that looks much like the one in Firefox (best used with a double monitor), tools to outline the different parts of your pages, to clear the cache,...

I've been playing with the beta1 for some time and it has showed pretty useful. The Beta2 has been available here since yesterday. I'll install it later today.