LibreOffice - Writer - Open-source cross-platform word processor.

Documize - Enterprise-grade wiki and knowledge management platform

Microsoft Word - Microsoft Word is a commercial word document processor for Windows.

HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.

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.

ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.