The first is the Web Developers Toolbar.

The toolbar adds many menus that aid in the development of websites. You can enable and disable JavaScript, images, CSS and cookies. You can also resize the window to show how a site will look on different screen resolutions plus a whole lot more. The toolbar also works on Flock and Seamonkey, and runs on any OS that can run these browsers such as OSX, Windows and Linux.

The other great plugin that I really love is Firebug.

"You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page" - Firebug.

There's a great part of this plugin that I use, which is the Inspect Tool. Have you ever made a site and wondered why your div is a few extra pixels wide? With Firebug you can identify the problem with ease. It displays the width, height, padding, border, margin and offset of each element in the page so you can see why your page isn't displaying properly. Plus you can edit pages on the fly so you can see how they look straight away.

These are really fantastic plugins. So a big shout out to Chris Pederick for the Web Developer Toolbar, and to Joe Hewitt, Rob Campbell and Justin Dolske for Firebug.