Joomla!
Joomla! The most popular and widely used Open Source CMS Project in the world.
JoomlaCode
JoomlaCode, development and distribution made easy.
Joomla! Template Club
The longest running and best known template club with a collection of over 80 themes.
phpBB3 Styles Club
It provides phpBB3 styles to compliment your Joomla site or even stand-alone.