Before you can enter any cheat codes, you'll first need to enable Cheat Mode. To do this, type this for your name:

gouranga

Now you should be able to enter any of the following cheat codes during gameplay.
Code - Effect
coolboy - $500,000
muchcash - $500,000
iamdavej - $9,999,999
segarulz - 10x Point Multiplier
danisgod - 200,000 Points
cutie1 - 99 Lives
gorefest - A Lot of Blood
tumyfrog - All Levels
godofgta - All Weapons
SCHURULZ - Double Damage
LASVEGAS - Elvis Clan
EATSOUP - Free Stuff
JAILBAIT - Get Out of Jail Free Card
MADEMAN - Have No Beef with Gangs
hunsrus - Invisibility
rsjabber - Invisibility
ARSESTAR - Keep Weapons after Death
nekkid - Naked People
losefeds - No Police
buckfast - Really Violent Citizens
wuggles - Show Location
fishflap - Small Cars
voltfest - Unlimited Electrogun
flameon - Unlimited Flamethrower