Etherpad

Etherpad is a highly customizable Open Source online editor providing collaborative editing in really real-time. Etherpad allows users to edit documents collaboratively in real-time, much like a live multi-player editor that runs in their browsers. Users can write articles, press releases, to-do lists, etc. together with 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.

(Source: Adapted from http://etherpad.org/#about)