Internet Explorer Plugins

• Debugbar for IE
  A great free tool that acts as a debuger. Nothing will rival firebug but this tool is probably one of the best for IE debugging.
• XRAY
  XRAY is a bookmarklet for Internet Explorer 6+, and Webkit and Mozilla based browsers (including Safari, Firefox, Camino or Mozilla). Use it to see the box model for any element on any web page. (Thanks Josh)
• Virtual PC | 2007 Version
  A great way to test older versions of Internet Explorer is via virtual machines.