You'll be able to enter these codes in the Passwords option (inside Extras) after finishing the game.
1aytmmymhk: 1911 Custom (powerful pistol)
Abraham: "Mobs Alive" iPod tune
Aottrykmyn: Altair Costume
Deskyhstyl: Desert Eagle
George: "Gekko" iPod tune
Jmsotsynrn: Type 17 Pistol
Mekakorkkk: Mk.23 Pistol
Mgo2play: "Subsistence Action" iPod tune
Mnsoymsyhn: Mosin-Nagant sniper rifle
Pkhhnwhsjt: Patriot (mega powerful, unlimited ammo and no reloads)
Skynytktjp: Mk. 2 scanning plug
Theodore: "Midnight Shadow" iPod tune
Thomas: "Desperate Chase" iPod tune
Tshsniammr: Thor .45-70 (one-shot-kill weapon)
Ammo and Bandana
After finishing the game once, Debrin will have new sets of ammo, grenades, and bandanas to buy.
Complete the game once to get Big Boss Extreme. You can also start a new game over your old saved data to have a lot of your equipment and points carry over.