Super Mario Kart
Contributed by Richard

At the same time press all the buttons a, b, y, x and you will be small. 
Doesn't work in battle mode.

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Contributed by-The Unknown Hello

Shrinking Racers

On the driver select screen, highlight your driver and simultaneously
push Y and A.
Note: This will not work in time trials or battle mode.

Special Cup Time Trials

Enter the time trials and select your driver.  Then push Left on the
control pad to move the cursor beside "mushroom cup".  Then push
L,R,L,R,L,L,R,R and A. "Special cup" should appear below "Star cup".