I think I may have finally found a solution – http://www.debugbar.com/
Some of the features…
- See every Ajax request, the response headers, and the source code.
- View all the linked and inline scripts, html, and css in any loaded page.
- HTML check – for validation
There are also some nice extras in there, such as the color picker, window resizing to preset sizes, and the ability to e-mail screen shots when an error occurs.