Firebug

This is one of those tools that are indispensable in every web developers toolkit. It’s a great tool for debugging interfaces. You can actively step through the DOM elements. It also allows for dynamically updating the CSS and HTML. But one of the greatest features for me is the displaying of asynchronous JavaScript calls. This makes debugging Ajax applications a lot easier.

FirePHP

This tool integrates nicely with firebug. It makes it possible to show PHP debug information on the firebug console. So the debug data doesn’t interfere with the page your working on. I have to admit. I don’t use it very often. But it’s still a great tool.

Greasemonkey

An extremely power full tool. That let’s you add scripts to automate tasks. Or to trigger certain attack vector’s :)

NoScript

If you’re scared of malicious JavaScript or just want to block those annoying ads. Then this is a great tool.

YSlow

Amazing tool from Yahoo. it let’s you analyze the page load process. It’s great to
track down problems with slow loading pages. Or just a tool to help with optimization.

Event Spy

A nice tool to track DOM events.

Header Spy

Want to know what the web server actually sends back? A great tool for analyzing web traffic.

Live HTTP Headers

Watch HTTP headers send between your browser and the web server.

Measurit

A virtual ruler to use inside your browser. It’s always good to be able to visually measure something

Modify Headers

Gives the ability to change HTTP headers

Tamper data

A nice tool for editing headers and post data. Great security tool for testing webapplications.

Technika

This is an awsome tool from the GNUcitizen project. It adds an active scripting envirement to the browsers. In which it is possible to script almost everything. From the current page to the browser chrome. This add-on acts as the base for other tools. For more information head over the GNUcitizen.

Zend Studio Toolbar

The toolbar that comes with Zend Studio or Zend Studio for Eclipse. It add some PHP IDE features to the browser. Like debugging and profiling. Just a shame that it’s a toolbar and not some status bar icon. The toolbar takes up a lot of space.

LeetKey

This tool adds some text transformation features to the right click menu. When enabled it’s possible to transform / encrypt text with the following encoding / encryption (L337, ROT13, BASE64, HEX, URL, BIN, DES, AES, Morse code)

Unicode Input Tool / Converter

Conver hexidecimal || decimal entities or characters to unicode.