Best open source e-commerce platform?

Anyone have any recommendations for the best/easiest/most customizable open source e-commerce platform?

I've created some sites using PayPal, Google Checkout and Cartweaver with some success, but I'm looking around at different open source solutions.

Seems like ZenCart and OSCommerce are some of the "best" and I've read more comparisons on the two of those than any other.

I'd like to do a site that functions similarly to www.piercedfish.com Completely different products, but I like their customized layout/look and the functionality is very straight-forward and intuitive. Looking through their site source, I can't tell if they're using a particular package...

Thanks for any advice!!

edit: also, it's important that the software be search engine friendly. :)

That's definitely a zen-cart site. Zen cart has lots of options but has a very messy structure to build upon. If you've got a great understanding of html, table based layout, and PHP then you shouldn't have much trouble making a theme for zen-cart.

OSCommerce I can't really comment on since I used it once and hated it. Didn't even finish with it and I found zen-cart. But, many people prefer OSCommerce to Zen-Cart

I also like wordpress and the wp-shopping cart plugin. It's pretty customizable, simple to use, and I'd say semi-open-source since there is a free and paid version. You're not supposed to modify the core of the free one I believe. It's made by a company called instinct.

There are some good ones for Joomla and Drupal too that I've heard of so you might check into building a cart in a CMS.