Firebug is an essential addon for ALL web developers. If you don't know about it, you are in for a treat.
Firebug incorporates some amazing options, such as live CSS editing, HTML, Javascript, and DOM viewing/editing. It is a powerful and extensive tool that should be in every web developers toolkit.
Chrome: https://chrome.google.com/extensions/detail/bmagokdooijbeehmkpknfglimnifench
Firefox: https://addons.mozilla.org/en-US/firefox/addon/1843/
