Unlock Black And White Mode
     Beat the game on Easy difficulty to play the game in black and white.

Unlock Infinite Ammo:
     Beat the game on Normal difficulty.

Unlock Authentic Mode:
     Beat  the game on Hard difficulty to play with no checkpoints, no
saves, no icons and no reticules.

Unlock Super Squad:
     Beat the game on Authentic Mode to unlock the super squad cheat.