Magento (http://www.magentocommerce.com/) is an extremely powerful & flexible e-commerce platform with much room for future growth. It uses the most up to date ecommerce technology with no monthly charge associated to it. The system gives you full control over all aspects of your store as well as detailed reports on your sales and customers.
The Magento software has been developed to be search engine friendly. Magento e-commerce is used by such major brands including Adidas, Nokia, North Face and Ford (http://www.magentocommerce.com/showcase), reason being is that they offer great support and a wide list of features (http://www.magentocommerce.com/product/features).