Free Etherpad - open source online editor providing collaborative editing in real-time
Etherpad is often embedded into team chat type services like Slack to allow for many people to simultaneously edit the same document. This could be to collaborate on getting a business case done, drafting a policy, etc.
What is different from Google Docs or Microsoft, is that it is fully open source, you can embed it elsewhere, or you can host it internally within your own company free of charge.
See more on Etherpad at http://etherpad.org/
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, ...