At the credits screen type "bghunters".

Enter these codes on the screen where you raise your weapon.

Code          Effect
bgbarbecue    Shoot with a rocket launcher
bgpredator    Infrared vision
bgsniper      Stops the gun from moving
bgweather     Changes the weather

Enter these codes on the map screen.

Code          Effect
bgshow        Shows animals on map
bgtrack       Stealth mode
bgurine       Attracts game to you
bgvampire     Show blood trails

Enter these codes at the main menu.

Code          Effect
bgnoah        More animals
bgbullwinkle  Hunt moose only
bgyogi        Hunt bear only
bghorns       Hunt big horn sheep only
bgelabong     Hunt elk only
bgsteroids    Hunt trophy sized game only