Cheats for the shareware release:

 Enter the command console (press [~]) then type

 god             - Enables god mode
 give 3          - Gives you weapon 3..8
 give s 255      - 255 shotgun shells
 give c 255      - Full battery charge
 give r 255      - 255 rockets
 give n 255      - 255 nails
 give health 999 - 999 health
 impulse 9       - Gives all weapons and keys
 impulse -1      - Quad power cheat
 impulse 11      - Gives you the jewels. 1 each time its executed. Do
                   it 4 timesfor all
 impulse 255     - Quad damage
 fly             - Enables flying mode; you will need to bind some
                   controls for it to be usefull.
                   try  bind _key_ +moveup (and +movedown)
 noclip          - Turn off clipping
 map E1Mx        - Go to map
 colorxx         - Changes player clothing color; xx is a range of
                   colors from 00 to 13
 gammax.x        - Alters screen brightness; x.x is brightness level
                   from 0.0 to 1.0
 name            - Changes player name
 status          - Displays info on the current game
 sv_gravity      - Gravity; xxx represents a number from 00 to 850,
                   where 00 equals no gravity and 850 is normal
                   gravity
 crosshair 1     - Cross wires on
 crosshair 0     - Cross wires off
 notarget        - Invisibility
 map start       - Jump to the start
 map end         - Jump to the level end boss

 How to enter Nightmare Skill Level

 1. Choose any skill setting by going through one of the first three
    doors.
 2. Go through the door for Episode 4 (the set of stairs on your
    right).
 3. Step into the water and let yourself sink.
 4. Quickly walk backwards. When you pop out of the water, you'll find
    yourself in the rafters.
 5. Walk left along the beam, then go through the hole in the wall.
    Follow the corridor, and enter the teleporter.
 6. Select an episode to play by going through the door of your
    choice.