Apple iWork - iWork is an office suite by Apple.
Greenboard - Generate beautiful static api documentation
Airborn - Create and edit documents online without sacrificing your privacy
Archbee.io - Archbee is a developer-focused product docs tool for your team. Build beautiful product documentation sites or internal wikis/knowledge bases to get your team and product knowledge in one place.
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.
You Need A Wiki - Create a wiki with Google Docs