Write Together - Learn to write, together, one day at a time.
Authorly - Read | Write | Explore - Connecting Authors
Typely - A sensitive, unobtrusive and reliable tool for any writer, newspaper editor, teacher, blogger or student STA
Kludd.co - Everyone writes.
Cowrite - Platform for creating stories collaboratively
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.