I find it interesting that so many people name a significant number of addons that Web Developer toolbar does for you. While they are all great plugins and might have a couple extra options, not sure if its worth it to have those plugins (for instance, MeasureIt)
Here is my list:
- Web Developer Toolbar (if you're a web developer [period])
- FireBug (essential for javascript/AJAX debugging)
- Google Page Speed (similar to YSLOW but it's from Google, very useful)
- ColorZilla (great color picker)
- HTML Validator (saves a bunch of time loading w3.org's page)
- Tab Mix Plus (anything and everything to do with tabbed browsing)
- Screengrab (if you use screenshots at all, use this)
- Google Toolbar (SEO)