This is a tough one. I have seen Etherpad [http://etherpad.org/] being used a lot. The there is Zoho suite of apps [https://www.zoho.com/] but I am not sure if they really match your requirements completely.
For writing and other docs, I personally use Dropbox but I know that is not the same as working in the browser itself.
Etherpad, Ethercalc and Mailpile (and more!) are all available on top of Sandstorm [https://sandstorm.io/apps/] - which basically makes it easier to manage them all.
For writing and other docs, I personally use Dropbox but I know that is not the same as working in the browser itself.