New Super Mario Bros. Cheats

If you beat a level with a time left of x11, x22, x33, x44 etc you get fireworks and the original mario level completion song will play.

The mushroom houses at the start of each world (the point with an arrow at the start) differ determining on the time you get on a stage.

If you get x11, x22, x33 on the timer, a Red Mushroom house appears, which generates items.
If you get x44, x55, x66 on the timer, a Green Mushroom house appears, which is a 1up minigame.
If you get x77, x88, x99 on the timer, a Orange Mushroom house appears, which lets you gain a Mega Mushroom into your inventory, thus overriding anything in it.