www.oscmax.com

Its not an add-on, You install this instead of osCommerce

its osCommerce but its been taken by a community and made better it seems. easier to edit the look - just have to change 2 files (one is the .css file) and the whole site looks different.

it also comes with some useful modules already installed.

have a look on there web-site they have a full list of features and in the forum is a link to a live demo site.