uhh... all of them? can't remember which ones are active off the top of my head but have done delta, usair, 2x american, united, probabably another one i've forgotten.
you have to pay attention and keep up with them or you'll get hit with yearly fees to keep most of these types open. MOST of them will waive the fee if you call to cancel when it hits your statement, just tell them you can't afford the monthly fee or you don't think it's worth the payment. hell one of them even paid me $10 to keep the card last month when i called to cancel lol.
airline cards also usually come with yearly perks like free lounge access or cheap companion tickets. the signup bonuses vary significantly so you don't want to get one at the wrong time. theres travel blogs out there that let you know the good deals, my girl keeps up with that stuff and lets me know when to sign up for one.