WildSnake (Game Boy)
Also known as: Super Snakey (JP)
This game has debugging material.
WildSnake is a Tetris-style puzzle game involving, well, snakes.
At the Main Menu screen, change the value at RAM address D19C to 01 (GameShark code 01019CD1; disable once in the menu) and press Start. You'll be taken to a setup menu which is tailored to whichever mode you were about to start. You can customize any of the challenges, including the stats of the endless modes. It includes how fast the snakes drop, how many snakes and eggs you need to get, the time limit, and how many points you receive.