Protohacks

(Windows Users) Unlocker

(Windows Users) Cygwin

Add Ons

Firebug - very useful for inspecting and modifying javascript, html, and css on any webpage. Extremely useful for debugging and tweaking your own sites!
Web Developer - Displays information and gives powerful options for several properties of any webpage.
Stylish - allows the user to add css to any webpage. Useful for making a site look the way you want it to look (at least on your end)
ColorZilla - eyedropper tool for extracting any color used in a webpage
Measure It - gives ability to draw a bounding box on top of the webpage that gives the exact pixel width and height. Can measure any element with this.
IE Tab - re-renders the current page in IE. Useful for ensuring your site looks and works the same in both FF and IE. (requires IE browser)
Opera Tab - same as IE Tab only uses Opera for rendering (requires Opera browser)
Adblock Plus - Won't help you with development but still a beautiful way to get rid of annoying page ads!
Adblock Filterset.G Updater - set of useful filters for Adblock