I’ve recently had the chance to use the Ecwid Shopping Cart and I have to say that so far I’m liking it. It integrates with WordPress very easily, and when you’re on the order page you can still see your website menu and header, etc., keeping the look and feel of your own site.
It has all the usual options that most shopping carts have, and there are plenty of upgrades that you can buy if you are looking for more functionality, but so far I haven’t had to buy anything, it’s all free to start with.
So if you’re looking for a free, simple, and functional shopping cart to use with WordPress, Ecwid gets my vote!