osCommerce or its variations if you need free shopping cart…
Here is a nice osCommerce template that I have had bookmarked:
Here is a cheap but nice shopping cart creator:
X-Cart is good for most things. You can try them at www.x-cart.com
So far one of my favorites for wordpress website is http://shopplugin.net/
I used Xcart before but am not big on it.. even though it is fully featured it used to be a pain to theme…. but xcart makes this new ajax css cart that I think has real potential and its free right now.. http://www.ecwid.com/
there are a few templates on here that handle it all as well
I have used http://www.uniecommerce.com/products/uni-emerchant.html it has unique features like multi merchant platform, can manage multiple store, multi payment gateway, etc. Other than this i would prefer you Magento which is meant for its standard professionalism.