The most popular eCommerce available. osCommerce, PrestaShop and OpenCart are standalone solutions while JigoShop and Woo are used in conjuction with Wordpress. VirtueMart integrates with Joomla to create your online store.
osCommerce is the most difficult and recommended for those familiar with PHP coding. The balance of the shops can be implmented by almost anyone; PrestaShop perhaps being the easiest to use.