Binary Land (NES)
This game has hidden developer credits.
Binary Land is, as you may have guessed, a game about ones and zeroes.
Hold A + B on both controllers at the same time, then press Reset. The title screen now has "KIKU. MEGU. LOVE STORY" in place of the options and copyright info. The same message appears in Challenger, another Hudson game for the Famicom. In addition to this, the penguins are renamed, and if either gets into trouble they will cry out for the other; normally, Gurin and Malon don't call out to each other when in danger.
A lesser-known Easter egg, Gurin and Malon can give birth to a smaller penguin (either pink or green) if they meet 16 times and their meeting time is between 1-3 seconds. The baby roams around the maze at random and poses happily with its mom and dad at the end of a stage, but will also cause the player to lose a life if it touches a spider.