Documize - Enterprise-grade wiki and knowledge management platform
WackoWiki - WackoWiki is a light and easy to install multilingual Wiki-engine.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.