One of our clients is an online motorcycle clothing and accessories business called Mototo.com. Mototo.com is powered by Magento, the popular open source ecommerce solution. We've found Magento to be great to work with. It provides a huge amount of functionality 'out of the box' and allows you to build an enterprise level ecommerce solution relatively quickly and therefore cheaply.
A piece of bespoke functionality we created for Mototo.com was a dynamic 'Bestsellers' page.