Inline CSS Editor
Inline CSS Editor (download) is an IE Explorer bar addin that's apparently based on the EditCSS Mozilla Firebird plugin. When it's enabled, it shows the applicable CSS for any page you browse to. You can then edit the CSS and watch the page update as you change it. Useful for debugging your own CSS, for analyzing someone else's, or for seeing what MSDN would look like if they gone with Webdings as the default font.
This really rocks!!
[Link from an old post on Jon Galloway's blog]