The HTML Validator extension is a great extension to compliment your Web Developer extension. The HTML Validator extension takes the current page you're looking at and validates the page and returns your results as to whether you have valid HTML or not.
Hover your mouse over the icon at the bottom and you notice how many errors you have in your HTML.
This extension is strictly an "extension" of what the Web Developer Bar provides to you and can give you an idea of how valid your competitors site it as well. :-)
Rating: Definitely a 5 out of 5.