There are a few no annual fee credit cards that do this. The only catches that they usually have a limit of $300/yr in cash back (but that is $6000 in gas and groceries), and you have to be sure that you buy gas from an obvious gas company like Exxon, if you buy it at 7-11 they don't recognize it as a gas purchase.

The one I have is the citibank dividend Platinum select. They even have a no fee balance transfer at 0% for a year on top of everything else.

http://www.citibank.com/us/cards/car...plat/index.jsp