Here is a list of single-player cheats. (Credit goes to Gamespy)

Enabling Cheat Mode

Load the game using the -DEV –TOCONSOLE command line parameters. Your command line should look like this: HL.EXE –DEV –TOCONSOLE
During the game, press the tilde key [~] and type sv_cheats at the console window to activate cheat mode. You must either die or pass through a loading screen before this code takes effect.

God mode

With cheat mode enabled, press the tidle key [~] and type god in the console window.

All weapons and ammo

With cheat mode enabled, press the tidle key [~] and type impulse 101 in the console window.

Giving Items

With cheat mode enabled, press the tidle key [~] and type give itemname in the console window.

Replace itemname with a word from the list below to give yourself items.

Ammunition

Ammo_357
Ammo_9mmAR
Ammo_9mmbox
Ammo_9mmclip
Ammo_Argrenades
Ammo_Buckshot
Ammo_Crossbow
Ammo_Egonclip
Ammo_Gaussclip
Ammo_Glockclip
Ammo_MP5clip
Ammo_MP5grenades
Ammo_RPGclip

Items

Item_airtank
Item_antidote
Item_battery
Item_healthkit
Item_longjump
Item_security
Item_sodacan
Item_suit

Weapons

Weapon_357
Weapon_9mmAR
Weapon_9mmHandgun
Weapon_Crossbow
Weapon_Crowbar
Weapon_Egon
Weapon_Gauss
Weapon_Glock
Weapon_Handgrenade
Weapon_Hornetgun
Weapon_MP5
Weapon_Python
Weapon_QuantumDestabilizer
Weapon_RPG
Weapon_Satchel
Weapon_Shark
Weapon_Shotgun
Weapon_Tripmine