Enter the following in the 'Passwords' section in the Options menu. You can turn codes on and off using the 'Bonus' option beneath the Enigma machine. :

DAWOIKS     Unlock all cheats
WHERERU     Everyone is invisible 
WHATYOUGET  'Silver bullet' (one shot kills)
BOING       Bouncing grenades
LONGSHOT    Any weapon can zoom
MONKEY      Gold star on current level
GLASSJAW    Enemies only die from head shots
TIMEWARP    Gold star on the previous level
URTHEMAN    One shot kills you
BULLETZAP   Bullets will not hurt you
HABRDASHR   Random hats 
TPDOMOHTON  Guns shoot tiny balls of light

Level Passwords:
ORANGUTAN   Level 2 
BABOON      Level 3 
CHIMP       Level 4
LEMUR       Level 5 
GORILLA     Level 6

Film Reels:
BACKSTAGEO  D-Day
BACKSTAGER  Needle in a haystack 
BACKSTAGES  Horten's Nest
BACKSTAGEF  A Bridge Too Far.
BACKSTAGET  Storm In The Port
BACKSTAGEI  Rolling Thunder

Invincibility:
Pause the game and press , L1, , R1, , L2, Select, R2

Infinte ammo:
Pause the game and press , L2, , L1, Select, R2, , Select

Special Medal:
Beat the game with gold medals on all missions to get the Medal of Valor.

Thanks to Revolution readers Donnie Lloyd, John MacDonald, Greeny, Sean, John, Fist Full of Dragon, Daniel Eichhorn, Chris, aj lolli, jojo101, MIDO, Cody Edwards, Emu, Straightbob, BoxCarRacer47, Daniel Calderon, Onti, sk8tilldeath23, seijin akki, Alex, Luke Stangarone, Drucipher, brooks, Justin Valdez, FatMan, Chris Dickerson, Nick Pascale, alex forney, Brad, Daniel Taylor, Darrel2k2, Tift, CAP. Michael, Brett, Jc, cody priestap, Symbiote spider, Chris Middlecamp, Rocky, David Stewart, Mark Rubin, Nick Garofolo, Mongoose Man, Yodabob, Joe Aeuger, Colin Holzmann, PS2GamerJustin, stILLsmokin516, SliPgrAeme, Chris HollyWood, Brian Doherty, Micah Woods, shane wood, Wes McGraw, T-no, Baby Moo Moo, Tim Hernandez, SPORTSTER083, David Hunt, Montana Rhoden, jakob2387, Jay Tazares, Snafu, LoneWolf, Hyegodfather, jing jinga dergan, Alex Hradek, Matt, Ken, isidro rocha, Jonathan Miller, zack wilson, Adrian Clifton, Jarrett, Leo Lara, Speed Racer 8, Dylan Davis, GreasyTacoPride, joe mayo, Don Anderson, Sirilo Garcia, Hammer, Patrick Thomas, Guido, Micah Troy Duncan, DEVIL DOG ARMSTRONG, Joe Mafia, Browdie Bellon and Private James Fitzmaurice!