Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
yWriter - Free writing software designed by the author of the Hal Spacejock and Hal Junior series. yWriter6 helps you write a book by organising chapters, scenes, characters and locations in an easy-to-use interface.