The
only card that provides AA lounge access is Citi Executive, discussion
HERE.
The
only card that provides UA lounge access is Chase Mileage Plus Club, discussion
HERE. (It's also available with JP Morgan Reserve, but to apply for the card requires a $10 million banking relationship.)
Two American Express cards provide DL lounge access. One is the Skymiles Reserve credit card, the other is American Express Platinum. Platinum also provides access to American Express' own
Centurion lounges, as well as access to lounges operated by
Airspace and Escape, plus new access to
Lufthansa lounges in Munich, plus a Priority Pass membership.