Costco has since closed this loophole so you’ll need a Costco Membership to get gas there.
A friend of mine just tipped me off on this trick where you can buy gas at Costco without a Costco membership. All you have to do is swipe an American Express card first, when you would normally swipe your membership card, and then swipe it again for payment. Afterwards, it will let you buy the sweet sweet nectar of condensed dinosaur bones at rock bottom prices without the $50/year membership. It’s that simple.
Normally, you have to swipe your Costco Card or the American Express Costco TrueEarnings card  in order to authenticate in their gas station systems. It appears that you can simply use any American Express card in the authentication phase because the American Express Costco TrueEarnings card is just an AMEX with a Costco bar code.
I’ve confirmed that this works in Maryland and others have reported success in other states (except those that require an attendant to pump your gas, like New Jersey and Oregon). Locate a Costco nearby  (check to make sure it has a Gas Station) and give it a try, Costco gas is usually many cents cheaper than local competitors.
(Photo by shindohd )