Press T to enter talk mode during game play.
Once there just type any of the following:

ammo = Gives full ammo
god = Gives god mode
guns = Gives all guns

God mode - god 
Full ammunition - ammo 
All guns - guns 
All weapons, ammunition, health, armor - kfa 
Scopes, silencer and camera zoom - mods 
Full armor - armor 
Full health - health 
Spawn snowmobile in snow levels - rosebud 
Invisibility - poltergeist 
Level - maphole 
Get Skillpoints (Level 3-4) - skillz 
Stronger explosions - baddaboom 
Show Position - pos 
Show Version - build

Access All Chapters
To open all chapters of the game, run REGEDIT and find the key: 
[HKEY_LOCAL_MACHINESOFTWAREMonolith ProductionsContract Jack1.0] 

Change the value of "EndGame" to 1. 
Note: Always make a backup before editing the registry.