There are several sections covering Half-Life cheats:
- Cheat console commands.
- List of all maps.
- List of all console commands.
- List of all console variables.

Do not use the cheat codes unless you have been stuck somewhere for longer than many days. They WILL spoil the game.

Start Half-Life using hl.exe -dev -toconsole. This will let you access the console using "~" key. To do this, create a shortcut to hl.exe (by right-clicking on hl.exe and selecting Create Shortcut). Then right click on the shortcut, go to Properties and add -dev -toconsole to Target: box so it reads "c:\whatever\hl.exe -dev -toconsole". Use this shortcut to start Half-Life.

You may have to type "sv_cheats 1" in console first.

If you are having problems getting these to work, read this post for some explanations.

The following commands are available in console:

• /GOD - god mode.
• /NOCLIP - walk through walls/fly.
• /MAP xxxx - go to map xxxx. Map names are of the format c#a#, (c1a1, c2a1, etc). We do not recommend using this command as it will serious distract the gameplay. See the map list for names of all levels.
• /NOTARGET - invisible to enemies.
• /GIVE xxxx - give item xxxx. The following items are available:
  • item_airtank
  • item_antidote
  • item_battery
  • item_healthkit
  • item_longjump
  • item_security
  • item_sodacan
  • item_suit
    
  • 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
    
  • 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_rpg
  • weapon_satchel
  • weapon_shotgun
  • weapon_snark
  • weapon_tripmine

Also, take a look at Console Variable List and Console Commands List