Mambo

Mambo is an award-winning CMS that is ideal for the smaller, less structurally complex site. It is well-established with strong support, and is straightforward to set up and maintain out of the box.

• Key features: WYSIWYG editing; independent administration interface; layout modification without editing code; menu customization; out-of-the-box PDF printing support.
  
• Strengths: Proven robustness; extensive support network; customizability; simplicity of installation through 4-step wizard; good documentation.
  
• Weaknesses: Limited security options; lack of workflow tools and version control; cluttered administration interface; inelegance of underlying code makes hacking difficult.

Official site: http://www.mamboserver.com/

Plone

The only one of our recommendations not to be based on PHP, the Python-built Plone CMS is an industrial-strength solution with user-friendly features, excellent scalability and the power of the underlying Zope CMS development platform on which it depends.

• Key features: Powerful editing tools; full-featured management interface; member development areas; excellent workflow utilities; customizable document "archetypes."
  
• Strengths: Accessibility and customizability of templates; scalability; large number of add-on modules; elegant, object-oriented underlying code, simplifying modification; fully featured out-of-the-box; strong support through the Plone Collective
  
• Weaknesses: Requires strong knowledge of Python and Zope to get the most from it; steep learning curve due to complex structure.

Official site: http://plone.org/

Conclusion

Ultimately the selection of a suitable CMS will depend on the specific requirements of the organization deploying it and the site it will host. By following a careful process of need assessment and feature comparison, it is possible to identify the system that will best meet those requirements.

Although this may seem like an unnecessarily long-winded process, it's worth bearing in mind the cost and upheaval involved in getting it wrong. The implementation of a CMS is not a trivial business, especially if the goal is to migrate an existing site with all its content into the new system. It is therefore well worth taking the time and trouble to get it right the first time.