JSwiki

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

Chrome Developer Tools

Firebug - The most popular and powerful web development tool for Firefox

Illuminations for Developers - Browser extension for Sencha ExtJS, Dojo Toolkit, SproutCore, qooxdoo, YUI3, Google Closure Library, and jQueryUI

Tilt 3D See the DOM as a 3D representation

Opera Dragonfly very similar to the Chrome Developer Tools

devtools-detect - Detect if DevTools is open