Press T during gameplay and type in the following codes:

Code
GRISWOLD
CAP IN MY ASS
NO CAP IN MY ASS
MPKFA
SPORK
KRUEGER
COUSTEAU
CLARICE
MONTANA
KEYMASTER
IDAHO
LARA CROFT
EVA GALLI
GOONIES
JOJO
ONERING
MARIO X Y
KEVORKIAN
MCGEE
STERNO
 Effect
200% Armor
God Mode
Disable God Mode
God Mode (One Unit Whole Blood)
Full health
Full health, but sets you on fire
Gives you the diving suit
Sets your health to 100%
All items
All keys
All guns
All guns with infinite ammo
No clipping mode
Reveals automap
Gets you drunk
Turns you invisible
Level skip (x=episode, y=level)
You die
You burn to death
Blinding effect

Map List

Use the mario code to teleport to any level:

Code
mario 1 1
mario 1 2
mario 1 3
mario 1 4
mario 1 5
mario 1 6
mario 1 7
mario 1 8
mario 2 1
mario 2 2
mario 2 3
mario 2 4
mario 2 5
mario 2 6
mario 2 7
mario 2 8
mario 2 9
mario 3 1
mario 3 2
mario 3 3
mario 3 4
mario 3 5
mario 3 6
mario 3 7
mario 3 8
mario 4 1
mario 4 2
mario 4 3
mario 4 4
mario 4 5
mario 4 6
mario 4 7
mario 4 8
mario 4 9
 Effect
Cradle to Grave
Wrong Side of the Tracks
Phantom Express
Dark Carnival
Hallowed Grounds
The Great Temple
Altar of Stone
House of Horrors (Secret Level)
Shipwrecked
The Lumber Mill
Rest for the Wicked
The Overlooked Hotel
The Haunting
The Cold Rush
Bowels of the Earth
The Lair of Shial
Thin Ice (Secret Level)
Ghost Town
The Siege
Raw Sewage
The Sick Ward
Spare Parts
Monster Bait
The Pit of Cerberus
Catacombs (Secret Level)
Butchery Loves Company
Breeding Grounds
Charnel House
Crystal Lake
Fire and Brimstone
The Ganglion Depths
In The Flesh
Hall of the Epiphany
Mall of the Dead (Secret Level)

Trivia

Many of the codes poke fun at movies or games. GRISWALD is a reference to the Diablo 1 blacksmith. KRUEGER, which sets you on fire, is a reference to Freddy Krueger (who died being burned alive). EVA GALLI refers to the movie Ghost Story (1981). ONERING is a reference to Lord of the Ring. MONTANA, which gives you weapons, is a reference to Scarface.