Broadleaf Commerce... It's free, open source, and enterprise-class. Version 2.0 is simple to set up and configure out of the box. It also allows you to customize, extend, and integrate in absolutely any way that you need. We were able to get a complex, highly customized site with a large number of custom features, including flash sales and perishable inventory, deployed to production in 6 weeks. http://www.broadleafcommerce.com