MiniVend is a freely-redistributable electronic catalog, otherwise known as a shopping cart. It serves any number of catalogs via the World Wide Web, allowing users to select items for placement on their shopping list, or cart, and then buy them by filling out an order form. Since MiniVend supports Secure Sockets Layer (SSL), the customer can securely put their credit card number in the form if you have an HTTP server that supports SSL.
MiniVend supports a host of features -- a complete list is available at its web home page:
http://www.minivend.com/minivend/
MiniVend is written and maintained by Mike Heins, but much credit goes to Andrew Wilcox, who originated Vend and created many of the underlying algorithms.
MiniVend is freely redistributable under the GNU General Public License, a copy of which should come with every MiniVend package. Basically this means that you can use it for commercial or non-commercial purposes for free, but you you have certain responsibilities if you modify and/or redistribute it. See the GNU GPL for more information.