What are some alternatives?
Koding - A new way for developers to work.
CodePen - A front end web development playground.
Codeanywhere - Codeanywhere is a complete toolset for web development. Enabling you to edit, collaborate and run your projects from any device.
JS Bin - Sample of the bin:
Codassium - A better way to conduct remote interviews. Collaborative code editor + video conferencing + code execution = Awesome interviews.
Plunker - Plunker is an online community for creating, collaborating on and sharing your web development ideas.