Quackshot

Unlimited lives:
Go to Dracula's castle after getting the Bubble gum blaster in Duckberg. Find the first stack of large barrels. Get the extra life from the last barrel on the second row. Leave and re-enter the castle to get the same extra life.

Pepper madness:
Pause game play after Donald eats a pepper. Press Up(4), Down and resume game play.

Hint: Maze level:
Follow 1, 5, 4, 4, 2, 6 in the maze level to reach the tiger.

Hint: Unlimited popcorn:
To get unlimited popcorn (which is very useful against Bosses because it scattershots the entire screen), go to Duckberg. Go to the screen where Gyro is located, then ride the power lines. Halfway through riding the power lines, jump onto the power line with all the popcorn. Then, ride the power line back. Exit the screen and return. You can do this as many times as desired.
Information in this section was contributed by D-Star.