CodePen - A front end web development playground.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Omnibug - Omnibug is a plugin for the Google Chrome and Mozilla Firefox browsers to ease developing and...
JS Bin - Sample of the bin:
HTTP Header Live - Displays the HTTP header. Edit it and send it.