My fav is Wordpress, initially a blogging CMS but now its too powerful, you can do anything out of it. My second fav is Joomla, this is also a super powerful OS free CMS based on PHP. It range from personal page to high traffic websites.
there are soooo many free ones out there. search sourceforge. I don't use a CMS on my personal web site, and neither does our Robotics team, however we know some other teams that do.
