OpenCart is an open source PHP-based online shopping cart system. A robust e-commerce solution for Internet merchants with the ability to create their own online business and participate in e-commerce at a minimal cost.
This is an excellent shopping cart software I ever used. I am hosting my www.daania.com using opencart. It's a very flexible open source. I could do a lot of customization by myself. Though it took some time initially, everyone with some knowledge on php an mysql can make customization without any hassle.