CodePen - A front end web development playground.
collabedit - Collabedit is an online and collaborative text editing program suited to programmers and ideal for job interviews, educational lessons, and team projects.
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Rustpad - Open-source collaborative text editor based on the operational transformation algorithm.
replit - Code, create, andlearn together. Use our free, collaborative, in-browser IDE to code in 50+ languages — without spending a second on setup.
Pi7 Collab Edit - Collab edit is described as “a collaborative, real-time code editor in the browser.