No more Mambo is topical nowadays. Joomla is a new name for Mambo.
http://www.joomla.org/
Read the FAQ to be cautious that Joomla will be on its own way and tends to be incompatible with future versions of its ex-Mambo incarnation. Joomla 1.0 is derived from Mambo 4.5.3, but 1.0 doesn’t speak for Joomla’s inconsistency and immatturity.
