Joomla!
Home of Joomla!
OpenSourceMatters
Home of OSM
php.net
Joomla! - Forums
Joomla! Forums
MySQL
The database that Joomla! uses
Ohloh Tracking of Joomla!
Objective reports from Ohloh about Joomla's development activity. Joomla! has some star developers with serious kudos.
Joomla! in Wikipedia
A Joomla Urban Dictionary