Web Developer Toolbar
Adds a menu and a toolbar with various web developer tools.
FireBug
Edit, debug, and monitor CSS, HTML, and JavaScript live in any webpage.
YSlow (For FireBug)
Analyzes web pages and tells you why they’re slow based on Yahoo’s rules for high performance web sites.
ColorZilla
Advanced Eyedropper, ColorPicker, Page zoomer.
Update (12/07/08 - 15:08)
HTML Validator
HTML Validator is a Mozilla [...]