Joomla! is a free, open source content management system which is a fork, or off-shoot, of another CMS called Mambo. It's cross-platform, written in PHP, and contains all of the features you would expect from a modern CMS including: RSS feeds, page caching, blogs, searching, and language localization. It was officially separated from Mambo in 2005 when some of the developers stated that Mambo went against traditional open source values, so Joomla! is supposed to follow the “totally free, totally open” mentality. So far, it's doing well.