MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
TiddlyWiki - a non-linear personal web notebook
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!
Nuclino - Nuclino is a unified real-time workspace for teams that brings all your notes, tasks, and docs together in one place.
Slack - A messaging app for teams who see through the Earth!