Oscommerce is an extremely popular open source e-commerce software package that is being used on literally hundreds of thousands of e-commerce websites worldwide. The base install is rich with features designed to help you manage your online storefront including customer management, member managment, product and product category management, international currency management, shipping options, sales tax configuration, and more.
As with many open source products on the market today, OsCommerce has a large development and support community that is actively involved in the development of expansion modules to enhance the functionality of the OsCommerce program package. There are numerous add-ons available to expand the base install as well as a forum to answer any questions that you might have such as question dealing with administration and operation of your online storefront.