I would like to know best free e-commerce plugin for WordPress.
There are plenty of wordpress plugins available in the marketplace for building a wordpress ecommerce site. And this plugin would be good for building your ecommerce store http://wordpress.org/plugins/wp-e-commerce/.
For more wordpress plugins visit, http://www.apptha.com/wordpress
See this list and decide what works best for your needs: http://vandelaydesign.com/blog/wordpress/ecommerce-plugins/
There are so many free WordPress eCommerce plugins available but in my view DukaPress and WooCommerce are the best free plugins. These plugins can be more than other premium plugins in features.
And you can check the WPMU's collection on eCommerce plugins here http://wpmu.org/21-e-commerce-plugins-and-shopping-carts-for-wordpress-3-0/