MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
TiddlyWiki - a non-linear personal web notebook
Anchor CMS - Free and lightweight blogging system
Confluence - Confluence is content collaboration software that changes how modern teams work
Textpattern - Textpattern is an elegant content management system that is free, open source software.