All weapons:
Pause the game, then press Up, Down, Left, Right, Left, Right, Left, Left.
Get all comics:
Pause the game, then press Up, Down, Left, Right, Right, Left, Left, Up.
Get encyclopedia:
Pause the game, then press Up, Down, Left, Right, Left, Right, Up, Down.
Level select:
Pause the game, then press Up, Down, Left, Right, Left, Left, Right, Right.
No blood:
Pause the game, then press Up, Down, Left, Right, Up, Up, Up, Up.
Unlimited ammo:
Pause the game, then press Up, Down, Left, Right, Up, Left, Down, Right.
Unlimited Necroplasm:
Pause the game, then press Up, Down, Left, Right, Down, Left, Up, Right.