Duke Nukem 3D
Type these codes during play.

DNCORNHOLIO
God mode (and unlimited jetpack)

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