I've always been a fan of Joomla and Wordpress. I haven't wrapped my head around Drupal yet, but I hear it's really great, and it seems to be popular, so that's definitely another option.
I second the recommendation of browsing this site:
http://www.opensourcecms.com/
It features nearly every CMS/platform worth using, and provides a front-end and back-end demo for every single one of them.