* [BetterExamples.js](https://github.com/willemmulder/BetterExamples.js) - An in-browser debugging tool that makes live examples a great experience and also offers a step-debug tool. Based on Esprima. - [Demo](http://willemmulder.github.com/BetterExamples.js/)
* [Chrome Developer Tools](http://code.google.com/chrome/devtools/docs/overview.html)
* [Firebug](http://getfirebug.com/) - The most popular and powerful web development tool for Firefox 
* [Illuminations for Developers](http://www.illuminations-for-developers.com/) - Browser extension for Sencha ExtJS, Dojo Toolkit, SproutCore, qooxdoo, YUI3, Google Closure Library, and jQueryUI
* [Tilt 3D](https://addons.mozilla.org/en-US/firefox/addon/tilt/) See the DOM as a 3D representation
* [Opera Dragonfly](http://www.opera.com/dragonfly/) very similar to the Chrome Developer Tools
* [devtools-detect](https://github.com/sindresorhus/devtools-detect) - Detect if DevTools is open