At the supermarket, they want you to bring your shopping carts back to the store or to one of the cart corrals, but typically the parking lot is full of shopping carts left wherever they happen to be. And you can't help but feel sorry for the poor guy who has to go out in the rain and herd them back to the store. For all that, at the end of a tiring shopping trip is the time one is most reluctant to repeat the long haul to the store with the empty cart, and there's never a corral nearby.
So for as long as I can remember, I've counted my Kart Karma. When I head in, I always try to bring in at least one cart with me, so that I make the lot no worse than how I found it, and hopefully better. If I really can't bring in a cart with me, I might bring my cart back, though I usually have a surplus of Kart Karma from times I didn't need to take the cart back out but I brought one in, or when I brought in two, or when I was near a corral so I was able to return my cart there easily (though that doesn't count as much).
I bet that poor guy who has go out and gather the carts isn't much impressed when I leave mine by the car, though, since of course he doesn't know that that's where I got it from. But it's still an everyday kind of ethics.