Drupal, one of the really strong CMS/CMF/CM-what-have-you out there.
It started out as a "simple" Content Management System and grew up so quickly to become a solid framework.
It's no secret that this very website is being powered by Drupal (check the footer area).
With every technology worth your while comes a relatively steep learning curve, and Drupal scares off quite a big number of strangers because of that.
But again, what's worth working with logically requires some time investment to produce something great.
This section contains a small part of things that might come in handy when using this incredible open-source project.