Etherpad

Etherpad is a highly customizable online document editor that allows for collaborative editing in real-time by up to thousands of real-time users. With Etherpad, you don’t have to send documents back and forth-- simply set it up, share the link and collaborate with co-workers, fellow students, or friends on just about any written document!

Etherpad provides all-access to data through a well-documented API and provides support for data export/import capabilities. It’s got an awesome set of features, including spell check, authorship colors, font styles, and more, and if these aren’t enough for you, there are plenty of plugins you can add to customize your experience.