I know for the fact that, if you are looking for a scalable shopping cart with no limits to the amount of categories,search engine friendly urls,flexibility,easy automated stock management, then Ubercart is a perfect ecommerce tool.You can have a look at Ubercart in function on this website in action.Ubercart is preloaded also with various payment gateways for ecommerce like paypal, 2checkout, moneybookers.com, authorize.net, worldpay and many others. Ubercart is a genuine tools for all types of ecommerce ventures.

I like www.zeuscart.com It is a open source and it is licensed under free GPL and it supports more advanced features to built your own online store.

Zuescart might be free. Hard to tell as I don't want to submit my personal contact data in order to collect yet another set of code.

Wonder how many of their "downloads" are just clickthroughs on the way to the newsletter enrollment trap...

HI
I do think that The Magneto is the best one currently as shopping cart platform (and free)
Although there are plenty of payment gateways which are offering their services with integration ready to Magento, Zen cart and OS commerce

Good luck,

I would highly recommend zeuscart which is the best shopping cart software to make your online business a easy and a profit earning one.http://www.zeuscart.com

OsCommerce commerce software. I haven't tried it myself, but it looks good. It's free and open source.