I just have/use the cards from airlines I travel often enough to actually use the miles. I do have some of my father's old ones from Pan Am, Midway, PSA though in a file drawer somewhere.
I am a member of the following because I use these airlines often enough for it to benefit me (I've gotten enough for several free tickets on one or two of them, and also have upgrade certificates I need to use).
America West Flight Fund
Midwest Express Frequent Flyer
United Mileage Plus