A couple of days ago, I got an email from the guys at DebugBar letting me know that they had just released their Beta1 for Debug Bar 5.0.  They’ve got a pretty good tool bar for doing web design and web development in IE.  Their DebugBar adds features like a DOM Inspector, an HTTP Inspector, a JavaScript Console and Inspector, an HTML Validator among other things.  It’s free for personal use, and the commercial license is easy to obtain.

There are a couple of other tools worth pointing out as well

Hope this helps!