Blue 9 Burger [92 Third Avenue, New York; Menu]
Although it's definitely no In-N-Out Burger, it's probably the most similar thing you can get on the East Coast. The menu is simple and the burgers are greasy, but as long you eat them while they are hot, you will definitely enjoy yourself. Make sure to get the "special sauce," which, like at In-N-Out, is some variation of Russian dressing. The fries are solid as well, making the combo a good deal.