A Sonic Cheeseburger includes American cheese, crinkle-cut pickles, chopped onions, shredded lettuce, and sliced tomatoes on a beef patty with your choice of mustard, mayo or ketchup (you can check out my previous review of one here). They normally go for around $3.50 or so (varying with location).
The deal is good at participating Sonic restaurants, while supplies last.
Here's the ad spot for the promotion (they seemed to have gotten their Groundhog Day's superstitions mixed up):
Photo via Sonic.