Joomla is an open-source content management system that was a spinoff of Mambo. We recommend Joomla over Mambo because it is a newer re-write of the application.

Joomla is designed to build websites with the typical top  and side menu.  You organize your contents into sections, categories, and articles.  Sections contain categories.  And categories contain articles.

Although Joomla does not have the concept of "tagging" built-in as Wordpress has, you can install the Joomla extension JTag to obtain this feature.

Joomla has collapsing and expanding menus built-in where click on a section menu item and it expands to show its categories
menu items.  And it even highlights the current menu item for you.

Joomla also has SEF url and file caching features built-in.  But you have to turn them on. We recommend that you turn on SEF as soon as you have installed Joomla.  And turn on file-cache just after development and about ready to launch.

Joomla runs on PHP using MySQL database. So you can download it from Joomla.org and install it on most Linux based shared-webhost that meets its system minimum requirements.  For the official documentations, see doc.joomla.org.  The Joomla website has a forum for you to ask support questions.

See wikipedia for more un-biased information on Joomla.  You can try out Joomla for a test-run at OpenSourceCMS.com.