Good online shopping cart software?

I’m setting up a web store for a friend, and I’m wondering if anyone has recommendations for good (ideally free) shopping cart software. I’ve used Zen Cart before and found it very awkward and hard to navigate, so I want to find something a little more user-friendly this time around…

I’ve also been looking into shopping cart plugins for wordpress as an alternative, so if you have recommendations on that front, I’d love to hear them too!