Useful: Drupal

Not sure why I have not noted before, the open source Drupal Content Management System (CMS). This is a full CMS that allows you to add and customise all elements. Themes can easily be created as well as add-ins, known as modules. It can be difficult to get used to, however it is getting better with each new version and there is help and tutorials available on the Drupal.org website. There is a mature API in place and is well documented. This is, as far as I am concerned, the best CMS out there.