Web: http://etherpad.org/

Etherpad Lite source code: https://github.com/ether/etherpad-lite

Wikipedia: http://en.wikipedia.org/wiki/Etherpad 

List of sites that run Etherpad Lite: https://github.com/ether/etherpad-lite/wiki/Sites-that-run-Etherpad-Lite 

YouTube: https://www.youtube.com/results?search_query=etherpad

Collaborating in really real-time

No more sending your stuff back and forth via email, just set up a pad, share the link and start collaborating!

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 our installation guide