Drupal - Drupal - the leading open-source CMS for ambitious digital experiences that reach your audience across multiple channels. Because we all have different needs, Drupal allows you to create a unique space in a world of cookie-cutter solutions.

Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.

Zenario CMS - zenar.io Tribal Systems (really powerful and flexible)

Anchor CMS - Free and lightweight blogging system

MODx - modx is a free, fast & flexible, ajax and seo content management system

Textpattern - Textpattern is an elegant content management system that is free, open source software.