Joomla is one of the world's leading Content Management System, and is currently powering the interface for the BC$ Ad network.
The CMS was voted best open-source project 2 years consecutively (2006 & 2007) by Packt's Open Source Best of the Web awards.
Joomla Extensions: http://extensions.joomla.org/
Cory Webb provides a basic "how-to" introduction for Creating Joomla Templates: http://www.howtojoomla.net/content/view/20/2/1/2
Joomla Design Templates at Joomlart (see JA Corona & JA Drimia): http://www.joomlart.com/templates_demo.php