Press ~ (tilde key) during game play to access the console then enter the following codes and press enter to activate them.

God Mode:
Iddqd

Invisibility:
IDBEHOLDI

ALL Weapons/Ammo:
Idfa

Unlock ALL Weapons/Ammo/Keys
Idkfa

Unlock Beserk Pack:
IDBEHOLDS

Unlock Bonus Level:
Idclev31

Choose Game BGM:
IDMUSxx (xx = Number)

Computer Area Map:
IDBEHOLDA

Entire Map:
IDDT

Invisible to Enemies Until You Attack:
FHSHH

Invulnerability:
IDBEHOLDV

Unlock Level Warp:
IDCLEV xx (xx = Level Number)

Unlock Light Amp Goggles:
IDBEHOLDL

Unlock No Clipping Mode:
Idclip

Unlock Radiation Suit:
IDBEHOLDR

Replace fists with Chainsaw:
Idchoppers

Show Position in Code:
Idmypos

Unlock Wolfenstein 3D Level:
Idclev32


View ALL Objects:
Enter IDDT two times in Map mode to see ALL objects in the form of little triangles.