The Delta Amex platinum card (not just the Amex platinum card), will get you a day pass for $25, I believe. The Delta Reserve card gets you a free membership, though it's rather pricey, $450. If you are Delta Diamond, you automatically get a free membership, and you can use the club no matter whom you are flying on, for as long as you want. That's a big reason why I always fly Delta (or Delta partners) when I deadhead, and get 20K MQMs on the Delta platinum card. Having access to clubs worldwide is an awesome benefit.