Cheats:

Press [Ctrl] + [Alt] + ~ to bring up the console window. The bold text below is what the code does. The regular text is the code that you put in the console window. 

God Mode: god
Skip To Last Level: doomhell
Freeze Everything: freeze
Full Weapons And Ammunition: giveall
Classic 1995 Version: givedoom95
All Keys: givekeys
Give Weapon: giveweapon_(weaponname)
machinegun
shotgun
plasmagun
bfg
chainsaw
rocketlauncher

Locker Codes:

Locker #017 = 347
Locker #023 = 531
Locker #038 = 409
Locker #039 = 102
Locker #054 = 142
Locker #079 = 364
Locker #103 = 259
Locker #112 = 538
Locker #216 = 624

Hints:

Enable Player Shadows:
Using your text editor to edit the "doomconfig.cfg" file in the game folder. Find the line:

seta g_showPlayerShadow "0" and change it to seta g_showPlayerShadow "1".

Third person view:
Using your text editor to edit the "doomconfig.cfg" file in the game folder. Add the following line to the file then press P during game play:

bind "p" "toggle pm_thirdperson"

Nightmare difficulty setting:
Using your text editor to edit the "doomconfig.cfg" file in the game folder. Find the line:

"seta g_nightmare "0" and change it to: seta g_nightmare "1"

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