Half-Life

To activate these cheats, you have to enter some command line arguments. Make the shortcut to Half-Life point to this - "C:\Half-Life\hl.exe -dev -console" replacing c:\Half-Life with whatever directory you installed the game to.

After running the game with those arguments, you can enter a console within the game by typing the tilde "~" key. If cheats don't work at first, you'll have to type in "sv_cheats 1" at the console and cheats should be enabled for the next level that loads.

Activate the following codes by opening the console, then typing each and hitting [ENTER] after each one.

/god: God mode/invulnerability.
/noclip: No clipping mode/walk through walls.
/notarget: Deactivates any reaction from enemies until they are shot at.
/give: There's a whole plethora of things you can get with this command...

Hexen

reveal-reveal map
bgokey-god mode
rjohnson-no clipping
crhinehart-all weapons
sgurno-full health
mraymondjudy-all keys
kschilder-sound debug
rrettenmund-ticker
braffel-get items
tmoore-get items
bpelletierxx-warp to level xx
ebiessman-pig mode
cstika-kill all monsters on current level
rambo-lose all weapons
quicken-type three times to commit suicide
plipox-change player class (x=0 (fighter), 1 (cleric), or 2 (mage))
init-restart level
pmacarther-version info
jsumwalt-coordinates
mwagabaza-run script (save before using this one)

martek-god mode
conan-all weapons
gatexx-warp to level xx
zeligx-change player class (x=0 (fighter), 1 (cleric), or 2 (mage))
angel-no clipping
mccoy-full health
greedy-25 of each item
janitor-all keys
puzzle-all puzzle parts
carnage-kill all monsters on current level
reveal-map cheat (use when looking at map)
deliverance-pig mode
where-coordinates

satan-god mode
nra-all weapons
locksmith-all keys
indiana-25 of each item
mapsco-map mode cheat (use when looking at map)
where-coordinates
casper-no clipping mode
deliverance-pig mode
sherlock-get all puzzle pieces
butcher-kill all monsters on current level
init-restart level
clubmed-100% health
visitxx-warp to level xx
shadowcasterx-change player class (x=0 (fighter), 1 (cleric), or 2 (mage))
~xx-plays CD track number xx, must have CD in drive
martek-type three times to commit suicide
conan-lose all weapons

HyperBlade

Type these cheats during gameplay:
MDMKSB: Increases attack ability, makes you more immune to attack.
SPICYBRAINS: Gives you upside-down characters.
POTATO: Reduces character size.
SHUIN: Enables a hidden team.
GORILLA: Gorilla characters.

Hunter Hunted

To use these codes, hit enter, then type the code, then hit enter again.
COLE: 100% health, all weapons, full ammo, and invulnerability
TREVOR: All weapons and full ammo
INVINCIBLE: I wonder what this could be...
SNELLINGS: 100% health
BLUE: Changes player color to blue
AVACADO: Changes player color to green
OCHRE: Changes player color to brown
VINCENT: Changes player color to gray
SAGE: Changes player color to bright green

Heroes of Might and Magic III

To use these codes, press [TAB] while in the game and type in the following:

nwcavertingoureyes: Gives hero 35 archangels.
nwcantioch: Gives hero tent, ballista, and ammo cart.
nwcigotbetter: Hero gains 1 level.
nwccastleanthrax: Hero gains maximum luck.
nwcmuchrejoicing: Hero gains maximum morale.
nwccoconuts: Hero gains unlimited move points.
nwctim: Hero gains all magic and 999 spell points.
nwcsirrobin: Lose current mission/scenario.
nwctrojanrabbit: Win current mission/scenario.
nwconlyamodel: Build all buildings in your castle.
nwcalreadygotone: See full secret item map.
nwcgeneraldirection: See the full map.
nwcshrubbery: All resources increased.

Heretic II (for demo)