OpenCart is an open-source shopping cart solution. So, if you want to run your own online shop, OpenCart is your best bet. Just like their slogan "It's simple to start and easy to run".
OpenCart offers a powerful store management. Clean and compact administrator dashboard, unlimited categories and products, downloadable products ready, product reviews and ratings, product options, attributes, and filters, easy customers and administrators management, multi-currency and multi-lingual, payment gateways, shipping methods, and so on.
And the best part of OpenCart is it's code architecture. It's so clean and simple. You can modify it as you need. You can even build a totally different web-based application with OpenCart, like a social networking system for example. What?? Yes, you can!
You can download the OpenCart via their website here. It's FREE and Open Source!