It's a free Cloud IDE that allows you to code up a website or node.js app, and then it gives you a custom URL from which you can run your creation, like
mycoolwebapp.glitch.me. It was created by Fog Creek - the people behind Stack Overflow, FogBuz and Trello.
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Pro Runs a server out of the box
Perfect for building web applications, be it client-only, server-only, or full stack.
Pro Private data are kept secret, even in public projects
Non-collaborators can't access private data (e.g. API keys) which are stored in a special file, and they are deleted when a project is forked.
Pro Persistent storage
Pro Multiple languages supported
Although the only official language/framework is Node.js, you can access the terminal and install other languages via apt-get (as long as it doesn't exceed the storage limit).
Pro VS Code integration
If you feel the built-in web editor is not sufficient, you can switch to VS Code and get the full power of its plugins. You still work with the files and server in the cloud, no extra setup required.