I've always had a "two credit card" plan, but this summer I found an offer so good that I thought it was worth adding another card to my stash. Pentagon Federal Credit Union's Visa Platinum Gas Cash Rewards card offers 5% cash back on gas purchases, and the reward is credited monthly. On our trip to visit Grandmom and Granddad this summer, (when gas was rreeaallyy expensive), we received nearly $100 back on our huge gas bill.
This card also offers 2% back on grocery purchases and 1% on other purchases, but for my family this is a "gas only" card that can be used at any gas station. While it has no annual fee, this 13.99% APR is higher than my NFCU and USAA cards and therefore I wouldn't ever want to carry a balance on this card. You might want to see if Penfed's offer would work for you.