Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Batflat - Lightweight, fast, easy, and free CMS.
Textpattern - Textpattern is an elegant content management system that is free, open source software.
Grav - The modern open source flat-file CMS
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
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.