Well, if your host carries it, Miva Merchant seems to be the
easiest to set up, use, and maintain, I know most who use it swear by it, there is usually a monthly fee;
http://www.miva.com/products/merchant/
Otherwise Red Hat Interchange is a free one,
http://interchange.redhat.com/cgi-bin/ic/index
otherwise ya could check out;
http://www.hotscripts.com
they have tons of scripts available...