Quake 
PC
Enter the console, then type in the cheats. 

Crosshair 1	Gives you crosshairs
Fly	Flying mode (D=up C=down)
Fov X	Increases peripheral vision (where x is your field of vision)
Give #x	gives weapon #x - 3=shotgun, 4=nailgun, etc..
Give C #x	Gives x # cells
Give H #x	Gives x # health
Give N #x	Gives x # of nails
Give S #x	Gives you x # of shells
Give R #x	Gives you x # of rockets
God	God Mode
Impulse1	Shotgun
Impulse2	Super Shotgun
Impulse3	Nailgun
Impulse4	Chain Nailgun
Impulse5	Grenade Launcher
Impulse6	Rocket Launcher
Impulse9	All Weapons
Impulse252	Pentagram of Protection (30 seconds)
Impulse253	Ring of Shadows (30 seconds)
Impulse254	Bio-suit (30 seconds)
Impulse255	Quad Damage mode
Kill	Commit suicide - restart level
map e#m#	Level Warp
Map End	Takes you to last level
Noclip	Walk through walls
Notarget	Invisible to enemies
R_Fullbright 1	Everything is bright
sv_gravity ###	sets gravity to ###
Version	Gives you version information

Nightmare Skill

Start in Normal skill mode. Go into the 4th episode but don't get into the time portal. When you fall out of the water, go to the console and type in "Fly". Go up , turn left, and enter the doorway...

or...

Step into the water but don't fall through. Swim all the way against the wall on the same side you enter the water (where the shield is in deathmatch). Now fall through and you will land on the rafter. Walk on the rafter right into the Nightmare entrance.

If you have a hard time getting anywhere on the rafters, use the NoClip code and walk through walls until you find it.

To be invisible to your deathmatch buddy but not the monsters, type: R_Drawviewmodel 0.

Thanks to Revolution readers Kasmir Gandalf, Azreal, Ben Ridley, Amanda Bearnik, Josh Riddell, Chris Psaros, Gwhite, Knut Erik Pedersen, Steven Hagle, Marc Lindenbach, Travis Shepherd, Nick, Michael G. Clawson, Ben Shotts, Richard Cunningham, and Aaron and James!