osCommerce (for “Open Source Commerce”) is an e-commerce and online store-management software program. It can be used on any web server that has PHP and MySQL installed. It is available as free software under the GNU General Public License.

Key Features
Compatible with PHP 4 and MySQL
Unlimited number of products and categories
Multi-lingual English, German and Spanish by default
Automatic web-based installation
Automatic setup of e-commerce payment systems: Authorize.net, 2Checkout.com, VSTORE.ca, iPayment.de, PayPal.com, PsiGate.com, SECPay.com, and TrustCommerce.com; Many other add-on ecommerce payment systems, off-line and on-line payment processing
Multi-currency support
Support for dynamic images
Supports physical products as well as product downloads
Web-based administration module
Database backup and restore
Temporary and permanent shopping carts
Secure transactions with SSL Secure Sockets Layer support
Many shipping options including by weight, price, or destination
Real-time shipping quote integration with UPS, USPS, FedEx, etc.
Zone-based shipping and free shipping options
Full tax functionality, different zones for different products