Many credit cards like Diner's Club, Amex Platinum, or Visa Black have their own programs.
For example, Amex Platinum gives you priority pass select + AA Admiral Clubs + US Airways Clubs + Delta SkyClubs. You need to be flying on those airlines to use those lounges (except for US Airways). Priority pass is going to have a big devaluation at the end of this month when they lose the Continental/United lounges.
I was flying jetblue and showed my amex in the US airways lounge and they let me in. So you don't have to be flying that airline to use the lounge with the platinum card. or else, i got lucky.