CubeCart – CubeCart offers version 3 of their software for free, but charges for version 4 after a free 30 day trial.  Offers bulk upload, inventory management, shipping modules and everything else you would expect from a fully operational online store.

Ecommerce Shopping Cart Software – A fairly new free shopping cart that puts a heavy emphasis on not having to know how to code, run databases or anything else.

IdeaCart – A free cart still in its Alpha phase, features fraud detection, newsletter support and more.

JadaSite – JadaSite is a bit unusual in that it’s Java based, as well as fully integrated with a content management system.

Magento – Launched in 2008, Magento is one the newest open source shopping carts on the market.

OpenCart – An open source PHP shopping cart.  It’s been a while since it has been updated, but everything appears to still be up and running.

osCommerce – One of the best known shopping carts out there, this open source cart enjoys a large community of plugin creators.  It has been quite a while since their last major release, v2.2 RC 2a, but they are in active development of version 3.0, which promises some major administration upgrades.

PrestaShop – An extremely new open source shopping cart that already seems fairly full-featured, and promises more to come with an area for expansion modules to be opened in the near future.

Quick.Cart.Lite – Quick.Cart.Lite is the free version of the Quick.Cart software and will allow you to build a simple online store.  Offers two more robust levels of the same program with more features.

Satchmo – Built on the Django framework, Satchmo offers highly customizable shop building with a long list of built in features such as Google Analytics incorporation, PDF invoice generation and more.

Ubercart – Ubercart is a full-featured shopping cart that also integrates with a Drupal site so that you can build a community around your store or featured products.

VirtueMart – A shopping cart built specifically for integration with the Joomla! and Mambo content management system.

Zen Cart – Zen Cart is an off branch of osCommerce that comes with several of the most popular modules built into it.