More information: http://shopplugin.net
Shopp is one of only two e-Commerce options mentioned here that is commercial-only and unavailable in a free version. The current price for a single-site license is $55, whereas a developer's license is $299.
The plugin also has a multitude of additional payment gateways and shipping modules available for $25 per add-on. Shopp is undoubtedly nice, but is also potentially very expensive.
Even without the optional add-ons, Shopp offers a plethora of features. Shopp shares WP e-Commerce's core philosophy of building the product catalog in the WordPress Dashboard, though you can also use shortcodes to add specific products to individual posts or pages. Like every plugin mentioned so far, Shopp supports both physical products and digital downloads. The shopping cart is also AJAX-enabled, meaning that it features drag-and-drop shopping. Keep in mind that WP e-Commerce also offers AJAX support with their optional DropShop 2.0 module.
Shopp...