Joomla! is a free and open-source content management system (CMS) based on PHP and MySQL (distributed under the GNU General Public License) and ideal for publishing and managing large amounts of content. A fork of the original open source CMS ‘Mambo’, Joomla! currently serves as the platform of choice for approximately 10% of CiviCRM installations. Joomla! was the second CMS to integrate with CiviCRM (2006).
On the feature curve, Joomla! ships with a fair number (i.e. no extensions required) that are suitable for what most web publishers' needs, including ACL management, advanced content parameters for article control, weblinks, contacts, custom fields and more. Joomla! has a mature ecosystem including free and paid extensions, and has a range of theming options available.
There are several Joomla! extensions (equivalent to Drupal modules and WordPress plugins) available to extend the functionality of the CMS as well as its interaction with CiviCRM. This includes plugins to sync CiviCRM groups with Joomla ACL groups, membership-based authentication, event listing modules, and various other user-integration plugins.
If you’re already familiar with content management and need a relatively straightforward way to publish and manage website content (in addition to leveraging a CRM), Joomla! might be worth checking out.