Open Source CMS with Custom Fields & API: ProcessWire CMS/CMF

There are many good CMS products out there, including Drupal, WordPress, Joomla, ModX, Expression Engine, SilverStripe, Concrete5, and so on. But having used them all, I think there is a real lack of products that accomplish all of these things:

Balance their complexity
Make sense from the get-go
Adapt well to existing designer/developer processes
Provide a powerful and easy-to-use API
Deliver an equally satisfying experience to the designer, developer and end-user

http://processwire.com/