WordPress - WordPress is web software you can use to create a beautiful website or blog. We like to say that WordPress is both free and priceless at the same time.
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
ApostropheCMS - Apostrophe is a powerful website builder platform built on an enterprise open source CMS, offering in-context live editing, visual design tools, and multisite enablement in a full stack JS environment.
Anchor CMS - Free and lightweight blogging system
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Textpattern - Textpattern is an elegant content management system that is free, open source software.