Start the game with the command-line shortcut of -DEVMODE. 
This will enable new cheat keys that can be used in the game.

Example: "C:\Program Files\Crysis\bin32\Crysis.exe" -DEVMODE 

Code           Result
---------------------
F1           - Toggle First/Third Person View
F2           - Jump to Next Checkpoint
F3           - Toggle Fly Mode/Ghost Mode
F10          - Jump to Previous Checkpoint
NUMPAD 1     - Give Missile Launcher, Shotgun, Alien MOAC
NUMPAD 2     - Unlimited Ammo
NUMPAD ENTER - DEBUG MENU

In addition to these cheats keys, you can now use the console by 
pressing ` (tilde) and typing any of the following commands: 

Code                  Result:	
-------------------------------
r_displayinfo 0     - Remove Debug/Devmode Info from Screen
g_godmode 1         - God Mode
i_noweaponlimit 1   - Carry Unlimited Weapons
i_unlimitedammo 1   - Unlimited Ammo
i_giveallitems      - Give All Weapons
g_suitSpeedMult = 2 - Faster Speed
V_goliathmode 1     - Indestructible Vehicles (Affects Enemies Too)