Etherpad allows you to edit documents collaboratively in real-time, much like a live multi-player editor that runs in your browser. Write articles, press releases, to-do lists, etc. together with your friends, fellow students or colleagues, all working on the same document at the same time. All instances provide access to all data through a well-documented API and supports import/export to many major data exchange formats. And if the built-in feature set isn't enough for you, there's tons of plugins that allow you to customize your instance to suit your needs.