Alternatives:

- Draft - http://draftin.com/ - Penflip - http://penflip.com/ - Typewrite - https://typewrite.io - StackEdit - https://stackedit.io/

There's also Quip, but its focused on mobile so doesn't fit well in this list. I did a pretty deep breakdown on two of four above (along with Editorially) earlier this year - https://zapier.com/blog/collaborative-writing-tools-editoria...