Mambo Open Source is a CMS (Content Management System) that allows you to build an entire website very easily. It is very easy to install and setup and has very low requirements (standard PHP and MySQL installations) and allows you to easily change appearence and functionality. Extra features, templates, language files etc are added by simply uploading zip files in the administration interface.
Mambos official homepage:
http://www.mamboserver.comMambos official development repository:
http://mosforge.netMambo news and articles:
http://www.mamboportal.comMambo community:
http://www.mambers.comMambo can be used for building everything on your website, it even has several forum systems developed especially for it. It also has integration ability with phpBB and now UBB.threads, with vBulletin possibly on its way.
Other available components: Gallery, Guestbook, User blogs, Java IRC, Games, Link systems, Classifieds and much more.