CodePen - A front end web development playground.
JSitor - Test your NodeJS, JavaScript, CSS, HTML online with JSitor code editor.
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
JS Bin - Sample of the bin:
replit - Code, create, andlearn together. Use our free, collaborative, in-browser IDE to code in 50+ languages — without spending a second on setup.
CodeSandbox - Online playground for React