I would recommend NOT saving your game if you activated ANY cheat codes.

Every cheat code in this guide I have tested to make sure they actually work, however if one does not work then tell me.

Player Cheats

GESUNDHEIT - 100 Health Instantly
TORTOISE - 100 Armor Instantly
IFIWEREARICHMAN - Gives you $250,000
MOREPOLICEPLEASE - Increase Wanted Level By 2 Stars
NOPOLICEPLEASE - Clears Wanted Level

AI Cheats

WEAPONSFORALL - Gives Pedestrians Weapons (They can carry any weapon in the game)
NOBODYLIKESME - Causes Pedestrians To Attack You
ITSALLGOINGMAAAD - Pedestrians Attack Each Other (And sometimes you)

Weather Cheats

ILIKESCOTLAND - Fog Thickness Lvl. 1
ILOVESCOTLAND - Fog Thickness Lvl. 2
PEASOUP - FOG Thickness Lvl. 3
SKINCANCERFORME - Clear Sky

Misc. Cheats

MADWEATHER - Speeds Up Clock
BOOOOORING - Slower Gameplay (Dialogue also slowed down, but radio remains normal)
TIMEFLIESWHENYOU - Faster Gameplay (Doesn't change dialogue speed)
BANGBANGBANG - Makes All Cars Explode
ILIKEDDRESSINGUP - Change Character
CORNERSLIKEMAD - Insane Handling (Pressing the horn button should make the car jump)
ANICESETOFWHEELS - Invisible Cars (Wheels, headlights, and taillights are still visible however)
GIVEUSATANK - Spawns a Rhino

Note: Cheats aren't case sensitive.
Note: Cheats I did not put on this list I either have not heard about, or could not confirm.