Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
MkDocs - Project documentation with Markdown.
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.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
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.
Docusaurus - Easy to maintain open source documentation websites