CodePen - A front end web development playground.
CodeShare.io - Realtime code sharing for developers
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
replit - Code, create, andlearn together. Use our free, collaborative, in-browser IDE to code in 50+ languages — without spending a second on setup.
Liveweave - Liveweave is a HTML5, CSS3 & JavaScript playground for web designers and developers
JS Bin - Sample of the bin: