Super Mario Bros.

Reset high score
On the title screen, press A, B, L, and R simultaneously.

All tips & tricks from Super Mario Bros. (NES) apply.

Super Mario Bros. 2

Suicide
Pause the game, then press L, R, and Select simultaneously.

All tips & tricks from Super Mario Bros. 2 (NES) apply.

Super Mario Bros. 3

SELECT your suit
(Note that this code may or may not work on your game. It is a leftover debugging code that works in some cartridges.)
In any level, simply press Select to change Mario's power. Just keep pressing Select until you get the power or suit you want; you can even get the Hammer Suit! To toggle Kuribo's Shoe, hold either Y, X, B, or A, then press Select.

For tips & tricks, see the Super Mario Bros. 3 (NES) tips page.

Super Mario Bros.: The Lost Levels

Reset high score
On the title screen, press A, B, L, and R simultaneously.

Worlds A - D
Getting to World A is much easier than in the original game. You'll go straight to World A after World 8-4 if you took a warp zone. If you didn't warp at all, after World 8-4 you'll get to play Worlds 9-1 through 9-4 before you go to World A.

All other tips & tricks from Super Mario Bros. 2 (Japan) apply.