Joomla! (http://joomla.org/) is the most popular open source Web CMS. It's been more than three years since Joomla! was born as a fork of Mambo (http://mambo-foundation.org/). Today Joomla! has an active community of more than 200,000 users and contributors. Joomla! has around 4,000 extensions and many themes.
Numerous high-profile sites use Joomla!. The code quality is good enough, but there is a steep learning curve. Many users complain about its template system. Also, the backend administration system could be simpler.
The Harvard University web site and the MTV's Quizilla web site are both Joomla! based.
But is Joomla! ...