Drupal

Drupal is a content management system (CMS) that allows users to build various types of websites. It was created in 1999 and is currently on version 5.1. Drupal is open source and has a large community of over 600 developers contributing to its core and over 800 contributing additional modules. It has a modular architecture that provides many features out of the box like blogging, forums, searching and multi-language support. While it has a learning curve due to its use of PHP, it is highly customizable and has a good developer community for support.