Web developer toolbars

Web developer toolbars — Special toolbar for web-developers. These toolbars usually provide one or a combination of debugging, CSS, HTML, DOM, JavaScript, and other tools which aid in web development. The toolbars are generally created by 3rd party companies, but do not effect the web browser's performance when not in use.

Here is list of popular browsers toolbars.

For Internet Explorer

Internet Explorer Developer Toolbar [1]
Firebug Lite [2]

For Firefox

Firebug [3]
Web Developer [4]

For Firebug

YSlow analyzes web pages and tells you why they're slow. [5]

For Opera

Opera developer tools [6]
Web Developer Toolbar [7]
Firebug Lite [8]