DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.

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.

TiddlyWiki - a non-linear personal web notebook

Airborn - Create and edit documents online without sacrificing your privacy

Pepperminty Wiki - An entire wiki in a single PHP file. Intended for small - medium sized wikis.

Bit.ai - Bit is a smart real-time document collaboration platform for teams to create interactive documents, manage digital content and track document insights.