I've been pretty happy with Zen-Cart. It's a fork of OSCommerce. The Zen-Cart folks will of course say it's better than OSC. Not having used it, I can't comment on OSC.
X-Cart isn't free and its templates are enormously complex. But, it seems to do just about everything under the sun. I have one client running it and they're happy. The same company also puts out LiteCommerce which they say is a bit easier to use. I don't have any experience with it to comment, though.