Draft is an online version control and collaboration tool for writers. https://draftin.com/

Fidus Writer

Fidus Writer is an online collaborative editor especially made for academics who need to use citations and/or formulas. https://www.fiduswriter.org/    


Etherpad is a customizable, open-source, online editor for collaborative, real-time editing. http://etherpad.org/    

Classical Text Editor

The Classical Text Editor is a word-processor for critical editions, commentaries and parallel texts. http://cte.oeaw.ac.at/