Have you ever stood behind someone in line at the store and watched him shuffle through a stack of what must be at least 10 credit cards? Consumers with this many cards are still in the minority, but experts say that the majority of U.S. citizens have at least one credit card -- and usually two or three. It's true that credit cards have become important sources of identification -- if you want to rent a car, for example, you really need a major credit card. And used wisely, a credit card can provide convenience and allow you to make purchases with nearly a month to pay for them before finance charges kick in.