No reward is more flexible than cash. The Citi Dividend Platinum Select MasterCard pays you back 5 cents for every dollar charged at pharmacies, gas stations, and grocery stores, and a penny for every buck spent otherwise. American Express's Blue Cash Card begins by paying you 1 percent on many purchases, and after you've spent $6,500 that year on the card, the rebate bumps up to 5 percent.

The cash usually doesn't keep rolling your way indefinitely. Citi Dividend, for instance, has a $300-per-year cutoff. But if you're a good shopper and find a deal, that $300 might pay for a flight as well as a car rental or a couple of nights at a hotel. Or birthday presents. Or the electricity bill. Or dog food ...