'''Etherpads''': For collaborating in real-time on documents, Etherpads are a great resource. They are also a good alternative to corporate-hosted and -provided services like Google Docs. Google's suite of sharing tools are popular, but it is important to remember that the data of the users and the content is on Google's servers reducing your control over your own data. 
Etherpads are also far easier for co-editing text than sending mails back and forth and using other asynchronous (i.e., not simultaneous) methods. The main thing you need to look for when choosing an etherpad is that it is hosted using an encrypted connection between you and the server (via https/SSL). A list of such etherpads can be found here: (https://github.com/ether/etherpad-lite/wiki/Sites-that-run-Etherpad-Lite).