#10 Quick Locale Switcher (useful for i18n testing)
#9 Html Validator
#8 Greasemonkey
#7 Lori (Life-of-request-info)
#6 MeasureIt
#5 ColorZilla
#4 Pearl Crescent Page Saver Basic
#3 DOM Inspector
#2 Firebug
#1 Web Developer toolbar
Without a doubt, this is the most useful plugin to have if you do any sort of web development.