DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
Airborn - Create and edit documents online without sacrificing your privacy
MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
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
Mellel - Home Page for Mellel: The word processor of choice for scholars, writers and long document writing. for Mac OS X and iOS.