Open Source App For Realtime Document Collaboration – Etherpad
Etherpad is an open source application for working on the same document in real-time with any number of users.
It is a web-based word processor that has formatting options, supports infinite undos and can import/export documents from/to multiple formats.
Each document gets a unique URL and users can simply go to that URL to start collaborating.
Users working on the same document can see each other's updates instantly, can chat with a built-in messenger.
The application has 2 versions, full and lite, where the full version comes as a debian package and the lite version completely based on Node.js (which is much easier to deploy).
There is also an API provided for easily interacting with the application and wrappers in multiple scripting languages are created for it.