Microsoft IE Developer Toolbar
I didn't even know about this and it has been out for about a month. Microsoft has heard the cries from web developers used to using Firefox's developer toolbar extension. While it is often pretty easy to validate your pages using Firefox, see how your block level elements are behaving, and look at the DOM of your page using the Firefox extension, it has been almost impossible with the awful lack of tools for Internet Explorer. They have finally addressed this.
I have found the toolbar to be extremely useful. The DOM inspector is wonderful in that it highlights the selected item if visible to indicate for which item you are viewing properties. If you have to build applications or websites using Internet Explorer at work, I hope you are designing for Firefox at home, no… I guess you always have to design for Internet Explorer, then you will love the new toolbar. I'd suggest that you download it and install it right away.