At the console (press ~) type:
GOD God Mode
NOTARGET No Target Mode
NOCLIP No Clipping
GIVE ALL All Items. 999 of each
GIVE BLASTER Gives 999 units of Blaster
GIVE BULLETS Gives 999 units of Bullets
GIVE CELLS Gives 999 units of Cells
GIVE JACKETARMOR Gives 999 units of Jacket Armour
GIVE SHOTGUN Gives 999 units of Shotgun
GIVE SSHOTGUN Gives 999 units of Super Shotgun
GIVE MACHINEGUN Gives 999 units of Machine Gun
GIVE GRENADELAUNCHER Gives 999 units of Grenade Launcher
GIVE ROCKETLAUNCHER Gives 999 units of Rocket Launcher
GIVE SHELLS Gives 999 units of Shells
GIVE GRENADES Gives 999 units of Grenades
GIVE ROCKETS Gives 999 units of Rockets
GIVE SLUGS Gives 999 units of Slugs
GIVE MINES Gives 999 units of Mines
GIVE NUKE Gives 999 units of Nukes
GIVE QUAD Gives 999 units of Quad Damage
GIVE INVULNERABILITY Gives 999 units of Invulnerability
GIVE SILENCER Gives 999 units of the Silencer
GIVE REBREATHER Gives 999 units of the Rebreather