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
Bloomfire - Let Bloomfire help you get organized! Organize your content, build your company knowledge base and help your employees to be more successful.
Confluence - Confluence is content collaboration software that changes how modern teams work
ZenDesk - Zendesk is a beautiful, lightweight help-desk solution.