Ether Pad Lite

Experiment on your own with this new Online Collaborative Editor DoingStuff.DonaldNoyes.20140814

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.
You don't need to set up a server and install Etherpad in order to use it.
Just pick one of publicly available instances that friendly people from everywhere around the world have set up.
Alternatively, you can set up your own instance by following an installation guide
- Site:
  - http://etherpad.org

I did a little test after installing it