Among them, most interesting and seems useful is Content Management Systems as shown in the article CMS Toolbox: 80+ Open Source Content Management Systems. Here's couple of the recommended one's.
Joomla - popular, award-winning CMS that will help you build powerful online applications.
XOOPS - extensible, easy to use; ideal tool for developing small to large dynamic community websites, blogs, portals and much more.
Drupal - equipped with a powerful blend of features, rich set of modules, very popular.
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