Some credit cards offer generous rewards for specific spending categories—groceries, gas, movies, home repair—so using certain cards for certain purchases makes sense. A free app called Smorecard will do the thinking for you. Download it, then indicate which cards you carry. (It won’t ask for card numbers, just names.) It uses a GPS to find you the best return based on your location. If, for example, you’re at the supermarket, it will show you which of your cards has a grocery cash-back program in play.