I like the WALD side-saddle wire baskets. Each basket holds a bag of groceries. You can also lay stuff accross the top and haul goods like a two-wheeled freighter.

If you are REALLY aggressive with the bungee cords, and stack grocery bags on top of each other, you can load five bags of groceries onto a WALD basket. You will, by the way, get noticed if you haul this many groceries on a bicycle.