Check out Plone, an open-source, community-supported Content Management
System built on top of Zope. A dozen national governments use Plone as
their institutional CMS. A good corporate example is Novell; most of
Novell's web presence runs on Plone instead of the CMS they own and
sell. Plone is built to cluster and scale, and it's basic operations are
rock solid.

Plone is a top-level framework that runs on top of Zope. Zope is a web
app server, so Plone runs Zope apps as plug-ins. There may be a hundred
(at least scores) of community-developed modules.

Plone has more to offer than Drupal, Joomla, or Alfresco, some of the
leading open-source CMSs, but there is a lot more to learn. Plone is
easy to set up and its basic functions are easy to manage, but if you
add a lot of functionality Plone gets complicated. Joomla, Drupal and
Alfresco are relatively simpler and have different look and feel. They
may have what you need, and you may prefer their simplicity/look/feel. 

You can check out reviews of other CMSs at:

http://cmsreview.net/

http://www.cmsreview.com/

http://www.cmswatch.com/


My favorite Plone resource sites:

http://www.plone.org/

http://planet.plone.org/

http://plope.com/

http://www.zope.org/

http://www.zopezen.org/

http://www.zopelabs.com/