Unlock all levels and difficulties: Enter bakersdozen as a profile name.
Unlock Black And White Mode
Clear Easy difficulty to unlock the "Old Movie" cheat. This basically allows the game to be played in black and white.
Unlock Infinite Ammo
Clear the game on normal difficulty to unlock infinite ammo. This cheat needs to be "reactivated" at the start of each level, if the player is killed, or he retries.
Unlock Authentic Mode
Clear the game on the hardest difficulty. This mode has no checkpoints, saves, icons nor reticules.
Unlock Super Squad
Play and complete the game on Authentic Mode to unlock the super squad cheat.
If you are killed three times in a row, the game will offer you a chance to heal any injuries your squad might have sustained during combat. To expedite this process, you can just throw grenades at your feet.
Speed This The Heck Up
Simply beat the game once on Easy. Then go to chapter select and play the very last level on a different difficulty level this will award you with that cheat.