Interchange. GPL

freshmeat.net: "Interchange is an advanced e-commerce platform. It enables you to build completely customized and flexible online shops, while utilizing the knowledge gathered in more than a decade of active development. It offers high level e-commerce functions, support for PostgreSQL, MySQL, and Oracle, templating, a complete shopping cart functionality, payment processing, inventory, tax and shipping calculation, discounts, Web-based administration, localization, event routing, SOAP-based RPC, a custom tag language, and the full power of Perl."