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The best open source e-commerce is osCommerce! Official Site
osCommerce is an Open Source based online shop e-commerce solution that is available for free under the GNU General Public License. It features a rich set of out-of-the-box online shopping cart functionality that allows store owners to setup, run, and maintain their online stores with minimum effort and with no costs, fees, or limitations involved.
osCommerce has attracted the largest community for an e-commerce solution that consists of over 112,000 store owners and developers worldwide with add-ons being contributed on a daily basis. To date there are over 3,600 add-ons available that have been created by the community to extend the features of an osCommerce online store.