For robust content management, my favorite product is Expressions Engine (for complex web sites such as news sites) and WordPress (yeah, it's a blog, but it's easy and does all the content management most simple sites need). Both are PHP/MySQL based, and easily extensible by plugins, custom code, API tags, or simply hacking the PHP source code. Drupal is also a contender that sits somewhere between Expressions Engine and WordPress in functionality.

Willing to go beyond PHP? Here are three alternative open-source CMS's worth taking for a spin:

  • Bricolage -- Perl based (read review)
  • Plone -- Python based, using Zope's transactional object database
  • Magnolia -- Built on the Java Content Repository Standard (JSR-170).

Willing to spend some money? Check out these commercial tools: