MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
Bitrix24 - Bitrix24 is a free (for small businesses) social enterprise platform. It is a united work space which handles the many aspects of daily operations and tasks. Create your own social intranet in minutes!
TiddlyWiki - a non-linear personal web notebook
Confluence - Confluence is content collaboration software that changes how modern teams work
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Nuclino - Nuclino is a unified real-time workspace for teams that brings all your notes, tasks, and docs together in one place.