Content management systems or CMS:
concrete5 - open source, free CMS
Drupal - an open source content management platform
ExpressionEngine - by Ellis Lab
Joomla! - mobile-ready and user-friendly way to build a website. Joomla! is free and open source.
TextPattern - a flexible, elegant and and easy-to-use CMS
Contao - web-based Open Source Content Management System, formerly known as TYPOlight
SilverStripe - create innovative websites and applications
Umbraco - open source ASP.NET CMS
The David Byrne Approach to Content & Community Management and the use of GIFs
Top 10 Content Management Systems - strengths and weaknesses