Quake Cheats
All versions
All these cheats must be entered at the Console. Get to the Console by pressing the Tilde key (~).
SV_GRAVITY # - Exchange # for a number. The lower the number, the lower the gravity. 
GOD - Invulnerability
NOTARGET - Monsters only attack you once you attack them
KILL - You commit suicide
IMPULSE 9 - Gives you all weapons, full armor and full ammo
IMPULSE 11 - Gives you a Rune. Use it once per Console visit
IMPULSE 255 - Gives you Quad Damage (temporarily)
IMPULSE -1 - Gives you Quad Damage and Pentagram of Protection (permanently)
GIVE # - Gives you a weapon, according to the number you put in place of the #: 
    1.Axe 
    2.Shotgun 
    3.Double-Barrelled Shotgun 
    4.Nailgun 
    5.Perforator (Super Nailgun) 
    6.Grenade Launcher 
    7.Rocket Launcher 
    8.Thunderbolt 
GIVE S # - Gives you # Shells for the Shotgun or Double-Barrelled Shotgun
GIVE N # - Gives you # Nails for the Nailgun or Perforator
GIVE R # - Gives you # Rockets for the Grenade Launcher or Rocket Launcher
GIVE C # - Gives you # Cells for the Thunderbolt
GIVE H # - Gives you # units of Health
GIVE HEALTH # - Gives you # units of Health
FLY - Lets you fly
MAP E#M@ - You warp to episode #, level @. Note the space between the MAP and the E#M@
MAP END - You warp to the end level, Shub-Niggurath's Pit
NOCLIP - No clipping. Note that this also accouts for the floor, so you can't go up or down stairs
CROSSHAIR # - Replace # with 1to turn the Crosshair on. Replace # with 0 to turn the Crosshair off.
Shareware version only
REGISTERED 1 - Game thinks that it's the registered version. You can now get the Thunderbolt. You can
    also enter the other episodes' slipgates but the game will crash if you do