Firebug - Firefox plugin helps analyze / troubleshoot web layouts.
Web Developer toolbar for Firefox - Firefox plugin that lets you validate, edit CSS of someone else's page
Colorblind filter - How do your images look to colorblind folks?
IE tab - a Firefox plugin that lets you rapidly switch to the IE rendering of a page
Sloppy - the slow proxy: View how your page loads over a slow connection.
WAVE - Accessibility report.