Debugger for IE - The Debug Bar.

Looks good at first impression, I'll update more on this post when I evaluate it.

The DebugBar V4.1.1 is an Internet Explorer plug-in that brings you new powerful features :

• DOM Inspector: View DOM Tree and modify tags attributes and css attributes on the fly to test your page
• HTTP Inspector: View HTTP/S request to check cookies, GET and POST parameters, view server info
• Javascript Inspector and Javascript Console: View javascript functions for easier debugging, see Javascript and AJAX code
• HTML Validator: Validate HTML code to correct and optimize your code and html size of your page
• And many more features: See page cookies, get pixel color on a page, make a page screenshot...
  http://www.debugbar.com/
  

For Scripting Debugging, there's a post in IE Blog:
http://blogs.msdn.com/ie/archive/2004/10/26/247912.aspx

regards,
choongseng