Sites often behave differently in IE than in other web browsers, so having debugging tools for this platform is important.
The Developer Toolbar for Internet Explorer versions 6 and 7 primarily provides a view of the DOM tree for a web page. Elements can be located visually and modified on the fly with new CSS rules. It also provides other miscellaneous development aids, such as a ruler for measuring page elements: http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038.
This free tool gathers an enormous amount of performance data for web applications and displays it in a number of ways for further analysis: http://ajax.dynatrace.com/pages/.