With the 5% cash back rotating categories on the major credit cards, it's not easy to maintain a steady 5% cash back. This My Money Blog post
has a good summary of the first quarter categories for Chase Freedom Card, Citi Dividend Platinum Select MasterCard and the Discover More Card. Don't forget that they require customers to enroll each quarter for the 5% cash back.
Don't forget PenFed
and Fort Knox FCU
both offer a Visa credit card with 5% cash back for gas.