Vendor Free

 
The internet is full of great resources for finding, assessing and demoing open souce content management systems.  There are far too many for me to discuss here, so i am going to mention a few that i have used or have heard others using with great success. The CMS Matrix (http://www.cmsmatrix.org/) has an enormous list of CMS's and some information about each.

MODx (http://modxcms.com/) classifies itself as a Content Management Framework, but it does indeed manage content, and it does it very well. It's simple and easy to manage and has some handy tools for content editing from the front end of the site, making the edits in the context of the page can be big selling point. One thing to consider is that although there are a handful of modules that come with MODx it is designed to allow you to create your own, and I haven't found as many online resources as there are for other CMS's.

Joomla (http://www.joomla.org/) is a recent fork of Mambo and is a very solid CMS with a lot of plugin options to make the user experience, on both ends, really great. The only draw back I have found with Joomla is that making modifications to components, and the CMS itself isn't quite as easy as I'd like to to be.

Drupal (http://drupal.org/) has been in the open source CMS market for a long time, and is very popular.