Here are some open-source CMS that are popular...
http://wordpress.org/
http://www.joomla.org/
http://drupal.org/
Wordpress, while obviously intended for blogging, with some tweaking can actually look more like a regular website.
Joomla is a widely popular free CMS, but seems klunky and unpolished in my view, but definitely worth checking out.
Drupal overall shows the most promise for many CMS needs ... nice interface, customizable, and pretty stable.
Ron