Magento is the flexible, feature-rich and secure eCommerce solution business owners need to propel their online business
Magento is open source software built by Varien, an industry leader and eCommerce provider for over 6 years. The software includes cutting-edge features such as single-page checkout, ship-to-multiple-addresses and a wealth of other high-end components that will improve conversions and elevate online businesses.

http://www.magentocommerce.com/