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 Super Mario All-Stars 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.

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