Scribus - Scribus is a desktop publishing (DTP) application.
Documize - Enterprise-grade wiki and knowledge management platform
Pages - Pages for Mac is a word processor that offers tools needed to create documents.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
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.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.