Enter the console, then type in the cheats.

Code                    Effect 
Crosshair 1             Gives you crosshairs 
Fly                     Flying mode (D=up C=down) 
Fov X                   Increases peripheral vision 
                        (where x is your field of vision) 
Give #x                 Gives weapon #x - 3=shotgun, 4=nailgun, etc..  
Give C #x               Gives x # cells 
Give H #x               Gives x # health 
Give N #x               Gives x # of nails 
Give S #x               Gives you x # of shells 
Give R #x               Gives you x # of rockets 
God                     God Mode 
Impulse1                Shotgun 
Impulse2                Super Shotgun 
Impulse3                Nailgun 
Impulse4                Chain Nailgun 
Impulse5                Grenade Launcher 
Impulse6                Rocket Launcher 
Impulse9                All Weapons 
Impulse252              Pentagram of Protection (30 seconds) 
Impulse253              Ring of Shadows (30 seconds) 
Impulse254              Bio-suit (30 seconds) 
Impulse255              Quad Damage mode 
Kill                    Commit suicide - restart level 
map e#m#                Level Warp 
Map End                 Takes you to last level 
Noclip                  Walk through walls 
Notarget                Invisible to enemies 
R_Fullbright 1          Everything is bright 
sv_gravity ###          Sets gravity to ###  
Version                 Gives you version information 

Nightmare Skill
     Start in Normal skill mode. Go into the 4th episode but don't get
into  the  time  portal.  When  you  fall  out of the water, go to the
console and type in "Fly". Go up , turn left, and enter the doorway...
or...

     Step  into  the  water  but  don't fall through. Swim all the way
against  the  wall  on  the  same  side you enter the water (where the
shield  is  in  deathmatch). Now fall through and you will land on the
rafter. Walk on the rafter right into the Nightmare entrance.

     If  you have a hard time getting anywhere on the rafters, use the
NoClip code and walk through walls until you find it.

     To  be  invisible  to your deathmatch buddy but not the monsters,
type: R_Drawviewmodel 0.