Like Magento, Zen Cart is an older open source e-commerce platform that was also a branch off of osCommerce. Despite being old, it still has a fair amount of active users. Zen Cart boasts many of the same features that other platforms have, but it has a less intimidating source code.
In addition to being relatively easy to understand and customize, Zen Cart also has a very active and friendly community. The forums are a great place to ask questions and the wiki has an extensive amount of information on all the things you could ever want to learn about the platform. Combine that with almost 2000 plugins and you get an e-commerce platform that has rightfully stood the tests of time.