Drupal is an open source (free) content management system (CMS) and development framework. It offers modules, themes, and translations. It is very popular and growing, so it is worth learning about and following. Warning: large learning curve or "cliff". InfoPeople hosted an introductory webinar on Drupal for libraries. Laura Soloman presented and answered questions. Check it out -- the webinar and handouts are archived.
Here are examples of websites created in Drupal:
YouTube videos about Drupal, including a weird Drupal Song: