Cheat Mode (demo cheats):
Press "T" key for console, top left corner of the screen, then enter either code depending on how good you think you are:

sanctuary   God Mode 
smeghead    Health potion refills
 

Press \ to display the talk feature, then type the following and press Enter to activate.

iamgod      God Mode
smoghead    Full health 
debug off   Disable debug mode   
debug on    Enable debug mode     
iamgoddess  God mode in last stage     
giants      Kill nearby enemies   
active drop Land with flight mode   
all weapons List weapon names in debug mode   
gimme ****  Obtain indicated item       
foghack     Remove fog     
switch      Switch view to nearest enemy   
enabled fly Toggle flight mode   
opensezme   Unlock doors   

Developer mode:
Hold down Shift while starting the game. Go to the "Developer Mode" tab and select the options you want. For other cheats, select the "Enable Developer Mode" box. Now during play, press \ to display the talk feature. One new cheat that Developer Mode allows you to do is gimme XXXX. Ex: Hit the talk key, then type gimme (see below):

health elixir
potion of life
great sword
battle axe
battle hammer
altir's mace
sword of flame
ice hammer
sword of ice
lightning axe
long sword
mace
mithril great sword
mithril axe
boomerang
fire boomerang
mithril hammer
mithril long sword
mithril mace
magic scimitar
magic short sword
scimitar
short sword
speed scimitarv mourn bringer
potion of invisibility
potion of invulnerability
flaming sword
long bow
arrows
lightning crystal
fire crystal
ice crystal
leather
plate mail
banded mail
dragon armor
rift crystal
runeblade
bell hammer of alwarren
rune cage key
scale mail
chain mail
tunnel entrance key
crusher room key
house key
studded leather
storage rune
elemental amulet
flaming arrows
poison arrows
explosive arrows
energy bow
energy arrow quiver
energy bow key
goblin access key
mithril throwing axe
ice arrows
mithril throwing hammer
jail key
dark scimitar
magic arrows
potion of invulnerability bronze
potion of invulnerability gold
potion of invulnerability dark
heavy bow
speed bow
heron's crystal
tuiri's soul crystal
dragon armor rune
lava rune
succubus mirror
mystic key
atimar's blade
dead man's key
rune of stone
grungle's hammer 
 

Thanks to Revolution readers Murad Idrees, Morbius Knight, Zeke of the Orks, and Colin!