The most popular ones are Wordpress, Joomla and Drupal, but there are many more out there:
MODx
Concrete5
CMS Made Simple
XOOP
Website Baker
Typo3
PHP-Fusion
Pligg
The list above is by no means exhaustive, it just gives you an idea of what's available.
Some Content Management Systems are designed for a specific purpose, such as Moodle, which is intended for e-learning and courseware.
There are also hosted CMS solutions such as Cushy CMS and Pagelime, which lets you set editable areas within any HTML page that your clients can update.