I can only recommend a paid one sorry but if you are later looking for a shopping cart that includes marketing autoresponders ,afilliate programs, lists and email blaster, digital delivery, recurring billing and much more, then PM me. I might put the link in my sig later if I am allowed to.
Avactis shopping cart is very easy to customize and integrate into template design, Paypal and 2checkout supported, you can download free beta version, click on SteveIrw signature for details
I think if you host your website with Crystaltech hosting company, they have a free shopping cart included. I think it's Zen Cart, but I'm not sure. Tony
I recommend OsCommerce. Really easy to work with and thousands of ways to customize it with mods and such. //Andreas
I think that oscommerce and zencart are good because they are free. We worked with osCommerce and found it very difficult from customization point of view. Of course if you a godd PHP developer, then it's ok, but it still requires a lot of time to customize.
OSCommerce is a nightmare to customize. Being ZenCart is based on OSC i'd have to assume the same thing about it.
You could always go with fully customizable external carts like www.romancart.com Its completely HTML based
Or you could go with an already "customized" already loaded OSC at http://www.creloaded.com/ I just check out that Roman Cart, it seems pretty cool.
My problem with OSCommerce lately has been the number of barriers they have which hurts conversions. I find myself having to basically Mod it beyond recognition. I wish they would introduce a standard OSLite which is stripped down considerably removing things like requiring people to create an account, etc Obviously if you sell things that are suceptible to chargebacks, its one thing but when you sell niche products, its a huge improvement to let people buy things with relatively little fuss.