From their website: "Magento is a new professional open-source eCommerce solution offering unprecedented flexibility and control. With Magento, never feel trapped in your eCommerce solution again."

Magento is made by Varien who are quite popular in the eCommerce world. It uses the Zend Framework. Take a look, it is still being worked on but you can download the Beta! I had a good look at the demo and it really looks nice!