An award-winning content management system (CMS), enabling the creation Web sites and powerful online applications using shared, open-source code.

http://www.joomla.org/

Blog engine

http://www.wordpress.com/

Product catalog and shopping cart software

http://www.virtuemart.net/

A powerful open source social networking engine

http://www.elgg.org/

eCommerce system that provides scalability, flexibility and features for business growth. Platform that offers merchants complete control over the presentation, content, and functionality of their online 'store-front'.

http://www.magentocommerce.com/