CodePen - A front end web development playground.
Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
JS Bin - Sample of the bin:
Materialize CSS - A modern responsive front-end framework based on Material Design