DebugBar

Awesome tools mentioned by Matt F:

Two programs for everyone to try out... http://www.debugbar.com/

1. Debug Bar – This is basically a cross between Fiddler and the IE Dev Toolbar, but it also has a JavaScript Debugger. It has some fantastic features like the Layout Inspector which will make debugging box model issues a breeze. The object inspector is presented in the UI as target icon, click the icon then any element on the page to inspect… very slick. The JavaScript debugger is fairly limited. It lets you execute commands in the immediate window but does not have any ‘watch’ functionalities. Certainly not as powerful as Firebug for Firefox. This is not a free application; an enterprise license is 1299 euros or $8478974894.94, at least the last time I checked the exchange rate.
2. IE Tester – This is awesome!!! A+++ to the developer. It allows you to render web pages with various IE rendering engines… It currently works with IE5.5, IE6, IE7, and IE8b1 engines. It is an Alpha release and is a little flakey especially in its awkward Ribbon implementation. But so far it is the only good way that I’ve seen to render pages in IE6 and IE7 on the same machine. IE Tester is freeware.