During play, pause and enter the following codes:

Master Code:
R, Black, Y, Black, L, White. You'll get all levels and bonus items.

X-Ray Graphics:
L, L, R, L, Y, X, White, Black, Black, White.

Freeze Graphics:
R, L, Black, White, L, Y, Y, Black, R, White.

Infinite Health:
Black, White, L, X, L, White.

Thanks to Revolution readers Alex Schmitt and MarkyMarky!