Quake

CHEATS....
To activate the codes listed, press the Esc key to enter the Quake Console. Once the Console appears, type in the code and press Return. To leave the Console and resume playing the game, press the Esc key once more.
COLORxx changes player clothing color; xx is a range of colors from 00 to 13
FLY toggles flying mode on and off
GAMMAx.x alters screen brightness; x.x is brightness level from 0.0 to 1.0
GOD toggles god mode on and off
NAME changes player name
NOCLIP toggles clipping on and off
STATUS displays info on the current game
SV_GRAVITYxxx gravity; xxx represents a number from 00 to 850, where 00 equals no gravity and 850 is normal gravity
VERSION displays Quake version info
IMPULSE 9 gives player all weapons and ammunition
F8 toggles monster tracking on and off
F9 toggles clipping on and off
F10 toggles god mode on and off

TIPS...

Tip: If an opponent has a Grenade Launcher, try to run circles around him. He'll end up doing more damage to himself than to you.
Tip: ALWAYS make sure you pick up your dead foe's backpack.
Tip: When you go through a slipgate, step back behind the gateway and wait for your opponent to pop in. He'll never know what hit him.
Tip: The only way to beat Chthon, the big bad guy at the end of episode one, is with a million volts of electricity. Luckily, the House of Chthon comes equipped with a device for doing just that. As soon as you grab the rune, Chthon rises from the lava. Quickly hotfoot it to the upper deck, and don't worry: as long as you keep moving, he can't get you. Lower the electrode pillars by stepping on the two switches; when they're both lowered, step on the third switch directly in front of Chthon. If you're quick enough, it'll put an end to him--and to the shareware version of the game as well.
Tip: The only way to defeat Shub-Niggurath, the Source of All Evil, is by telefragging her. See the spiked ball roaming about? That's the destination of the teleporter. Wait for that ball to move around the Quad damage tower and then into Shub-Niggurath herself. Hop in the teleporter, and--blammo!--you're a hero. The easiest way to do it is to avoid as many of the creatures as you can and scramble over to the Quad damage tower. Wait there until you see the ball about to slam into Shub-Niggurath, and then enter the slipgate. This slipgate will drop you right in front of the other teleporter. Charge forward to telefrag