osCommerce is a very popular open source E-commerce solution, and quite possibily the most deployed E-commerce solutions out there.  osCommerce in it's default installation is not all that great to look at., but provides a great frame from which to create a wonderful E-commerce solution.  osCommerce has a great community backing it, and there are thousands of contributed mods to customize your osCommerce site with.

There are several spin-offs of osCommerce including Zen Cart, osCmax, and CRE Loaded.  Most of these solutions are osCommerce default installs with a set of favorite contributions loaded.

Im currently in the process of creating a new site dedicated to using osCommerce and making it better.  You can watch its progress at http://www.oscommerce-ssl.com/index.html