Before using the cheat codes you must activate the cheat system. 
Simply follow these easy steps

1) At the Select Difficulty Level screen hold down the TAB key and press 
the CTRL key.
2) At the Select Player screen type: robisagod
You should hear a sound effect. Cheat mode is now active!
3) To turn on any of the following cheats, type the cheat name while in the 
game.
Invincibility = BEAGLE
Blood Code = GIVEBLOOD
Rescue All Hostages = RESCUE
Warp To The Next level = GETMEOUT
Display Hidden Areas = INFRARED
Turn On All Weapons ---- KICKASS 
Cut To The End Game Scene ---- GEEWHIZ

Easter Eggs
Secret Marine:
Hidden in the game is a secret character called Agent Squiggly.
To play as Squiggly type SQUIGGLY at the Select Player
screen and press the up arrow. Agent Squiggly is now ready for action!