osCommerce is the leading Open Source online shop e-commerce solution that is powered by a dedicated, strong, and ever growing community, and is released under the GNU General Public License.
Zen Cart has a very nice wide array of features based on Oscommerce but has gone its own path.. The ecommerce web site design program is being developed by a group of like-minded shop owners, programmers, designers, and consultants that think ecommerce web design could be and should be done differently.
Magento is one of the newer shopping carts on the block but has already attracted a large following. The coding is based on the latest PHP 5 object oriented coding standards and the Zend framework.