free, modulated and very good supported
I would be careful when first learning drupal as its a huge system and many ways to trip up but once you learn it then you are able to build great web sites and applications uising its set of api's and extending it via modules
Drupal is the most complete and flexible open-source CMS. And since it's PHP/MySQL, it can easily be installed about anywhere. From a simple personal blog to a full-featured company CMS or e-commerce platform, Drupal rocks!