Open Source Shopping Cart Solution

Magento: It's built using the Zend Framework and it's heavily MVC. If you're familiar with the Zend Framework, this might be a good solution for you. It's a lot more difficult and has a much steeper learning curve than PrestaShop.
TomatoCart: This is a fork of osCommerce 3.0 beta. The creators have tried hard to improve the platform, but osCommerce has a lot of rotten design and it's a mess to extend. I've tried creating a couple of modules and had many problems. There isn't much documentation to help you and the community is not very active.
VirtueMart: If you are familiar with Joomla, this might be an option. If you're not, you should stay away because you might have a few problems setting everything up correctly and integrate this component with your template.
osCommerce: Just stay away from this...

OpenCart