WooCommerce is a WordPress E-Commerce solution provided by WooThemes. It is free. How you ask? You end up paying money for the extensions that you plug into it. This seems fair as there is a ton of extensions for WooCommerce. Click here for a list of WooCommerce extensions.
Here is a video comparison between WooCommerce and WP e-Commerce by Press This (a wordpress plugin / theme youtube review series)
This is also an update about WooCommerce from the same reviewers
Keep in mind these videos do age as well as the plugin.