Type these codes while playing the game:

DNCORNHOLIO
God mode (and unlimited jetpack)

DNVIEW
Same as the F7 key 

DNHYPER
Steroids mode

DNITEMS
All inventory items and keys

DNWEAPONS
All weapons and ammo

DNKEYS	
All the keys

DNSTUFF
All weapons, keys and inventory items

DNSCOTTYxyz
Level warp; x is the episode number and yz the level (eg type 'dnscotty103' for episode 1 level 3)

DNSKILLx
Change the difficulty mode to x (x can be 0 to 3)

DNUNLOCK
Open all doors and unlock all locked doors

DNMONSTERS
Toggle monsters on/off

DNCLIP
Walk through walls