To display the console window to enter cheats press 'T'.

God Mode:
johnwayne

Skip Stage:
retreat
Infinite Ammo:
rocknroll
Full Weapons, Ammo and Health:
almighty

All Weapons:
onemanarmy
Restore Life:
didnthurt
Restore Ammo:
getsome

Restore Armor:
fullmetaljacket

Spectator Mode:
ghost