IE Developer Toolbar
I know this tool has been out there for a while, but it is highly recommended. It seems every time I refresh my ie7 beta I need to reinstall this plug in, get a new machine, or new job; I never can remember exactly what it is called and it takes me too long to find it.
This is a great BOON to a Web Developer that came from the desktop application side. I have learned HTML/CSS as I have went, and this IE plug in practically made it were I don't have to learn anything else about CSS. :)
It's not that great of a toolbar, but it really helps when trying to figure out exactly what I am doing wrong with my styles. Plus it has dynamic style changing and the IE Window is reflected immediately, no hitting F5 here!
Compared to some of the Firefox CSS plugins, this tool does everything that it does better than Firefox's, but Firefox has a really nice style sheet viewer. So you can see better how the cascading in style sheets really work.
http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en