CodePen - A front end web development playground.
Qoa - Interactive, elegant & intuitive command-line prompts 💬
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Snipper.ml - A simple snippet manager in the menubar
Liveweave - Liveweave is a HTML5, CSS3 & JavaScript playground for web designers and developers
Devhints - TL;DR for developer documentation