XOOPS

XOOPS is a PHP-MySQL CMS originally forked from phpNUKE. We found it clunky, poorly documented and infested with ginormous security holes and an illiterate and largely incompetent developer base, but your experience may certainly be different.

From the website:

XOOPS is an extensible, OO (Object Oriented), easy to use dynamic web content management system written in PHP. XOOPS is the ideal tool for developing small to large dynamic community websites, intra company portals, corporate portals, weblogs and much more. XOOPS uses a relational database (currently MySQL) to store data required for running a web-based content management system. XOOPS is maintained by a team of several hard-working volunteers working from all over the world with more than dozen official support sites. With a powerful theme system, both admins and users can change the look of the entire website with just a click of a mouse. There are also over 1000 themes available for download!

Yeah. Right. Well, anyway, here's the site:

Visit XOOPS