There is a new version of the IE Developer Toolbar from Microsoft!
"This Beta 3 version of the toolbar contains functionality and stability enhancements over previous versions, including:To view some screenshots of these new features check out the Microsoft IE Blog.
- Style Tracer: Right mouse click on a style value for an element and select Style Tracer to find the style rule that is effecting that value.
- CSS Selector Matches: View a report of all style rules set and how many times they are used on the current page.
- View Source: View the formatted and syntax colored source of the original page, currently rendered page, element or element with the styles that are effecting it."