Before telling about competitors I would like to ask if competitors for Magento already exist ;).
After Guido posted a comparison of shopping cart software I was surprised to see the amount of e-commerce platforms. Actually I expected to see a couple of well known software, like Intershop, OsCommerce or Virtuemart. Wikipedia showed a huge list of e-commerce software with their pricing, creator information, latest stable version date, updates and license information.
I think the amount of the software is not a threat for Magento, but more like an opportunity to present the retailers a shopping cart solution with the best practices of the e-commerce world. Let's take a short look at three of the open source competitors:
Another alternative for an open source shopping cart solution is Open Cart. This is a basic e-commerce platform meant for small business solutions. The interface looks like Magento and the menu structure too. You could say it’s a light version of Magento, like MagentoLight ;).
I heard from my colleges about the strange and too extended database structure of osCommerce. They were busy to migrate a complete database from osCommerce platform to Magento and not all data could be transferred correctly. Data migration is one of the most important issues the retailers have to think about before switching to another e-commerce platform.
This is a shopping cart extension of Joomla! CMS providing a user friendly, basic interface. During the time I was also developing websites, I made one website with Virtuemart. If I didn't know about the enhanced marketing possibilities, extensions (Magento Connect), and other great features of Magento, my choice should go to Virtuemart.