zencart or oscommerce but that is based upon experience from 3 years ago so maybe there are newer and better carts

Magento... bar none, the best shopping cart software right now. It's tweaked for maximum SEO, and it's just a well designed cart.

I would also agree with magento it was developed with extending functionality in mind. Users, even non-programmers, can install most basic extensions no problem. However, I think it works best with virtual or dedicated servers.

Oscommerce has not been substantially updated in a long time and there is no clear separation between business logic and the view (html). And extending it was always a pain and was unavoidable since key features like coupons and certain shipping options weren't included by default (at least when I used it last).

Pls search G for opencart, zencart, magento, prestashop, then you would get some feelings.

I didn't know there are free shopping cart software out there. May I know what language these software is using?

Oh.. I see. I'm not a php user, though. Is there a free shoppingcart for .net?

IMO the best is Magento.

i've been looking for this for a while.

Thanks to all who recommended.