This is actually my DH's idea. My new car has a hatchback, and DH suggested I get a plastic tote to put in the back. I wasn't sure about it, but it's turned out to be a very handy thing to have.
Most of the time it holds my emergency kit and my reusuable grocery bags. When I shop, it's good for holding those full grocery bags in one place. I've also found that the enclosed box helps my frozen and chilled foods stay cooler during the summer. (I've put one of those frozen blue ice things in too, to help things along. ) And if anything does make a mess, it's a lot easer to hose out the box than clean a carpet.
And if it's not too heavy, it makes unloading easier: Just grab the box and take it inside. (And if it is too heavy, I call DH ;)