First, enter the name GOURANGA to activate cheat mode. Now enter the following as your name before starting a level:

RSJABBER - god mode
MUCHCASH - start with $500,000 (save, exit, then restart your game)
GODOFGTA - all weapons, full ammo
GOREFEST - more gore
EATSOUP - free shopping
BUCKFAST - more aggressive peds
CUTIE1 - 99 lives
ARSESTAR - keep weapons after you die
FLAMEON - gives you a flamethrower
VOLTFEST - gives you electricity ray gun
IAMDAVEJ - gives you $9,999,999
TUMYFROG - gives you all of the bonus levels
SCHURULZ - double damage powerup
HUNSRUS - invisible
UKGAMER - gives you all 3 cities
BEMEALL - gives you all 3 cities
DANISGOD - gives you $200,000
MADEMAN - get all respect from 3 gangs
LASVEGAS - some people turn to "Elvis clan"
NEKKID - all people naked
FISHFLAP - makes all cars small