Take a look at opensourcecms.com. It's an extremely useful site that hosts demos of all the popular open source CMS's, with reviews and comments from other users.

In particular, the most common today seem to be;

• Joomla
• Drupal
• Textpattern
• Wordpress
• Mambo

Also, I've personally taken a look at MODx and sNews which seem to be a head above the rest, and might be worth checking out.