CodePen - A front end web development playground.
HTTP Debugger - Debug HTTP API calls to a back-end and between back-ends. Easy of use, clean UI, and short ramp-up time. Not a proxy, no network issues!
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
JS Bin - Sample of the bin:
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.