Best. Web. Development. Tool. Ever. Just install it.
This add on to Firebug allows you to view, manipulate, and delete cookies. This really should just be in Firebug, anyway.
Very nice screen shot tool. It can also do full page screen shots.
Bookmark syncing across computers and browsers. I personally use it to sync my bookmarks between Firefox and Chrome.
Displays validation errors for any page when you view source.