Joomla is in top 5 most popular open-source content management systems (CMS). Free licensed under GNU GPL license, it have large community of developers, users and many features including powerful content management (text/media/video/photos), user management – permissions per user and usergroups, template system (separates design from code), extension system (add new features without altering joomla core code), contact manager, menu, internationalization (translated in many languages), blogs, static pages, polls and many others. Check joomla features here. Many popular sites are powered by Joomla: Quizilla.com, Linux.com, Kontera.com and many others. It is written in PHP dynamic web programming language and requires database system like MySQL (others are supported too) in order to be installed. The latest version is 1.5.
Justhost as Joomla Host
Justhost have support of PHP and MySQL, and with Fantastico one-click installer cPanel tool you can install Joomla with one click. I have an account and on my test Justhost showed as a good Joomla Host.