Here are a huge list of open source php based content management systems.

XOOPS
Mambo
Joomla
Drupal
Yupi CMS
Wolf CMS
Website Baker
Typolight
Triton
plum cms
pligg cms
phpwcms
php-nuke
php-fusion
php micro CMS
OpenRat CMS
Limny
Limeware
Lemon CMS
Jaws
html-edit
Guppy
eliteCMS
CMS Made Simple