Type in the codes during gameplay. Case does not matter:

BAGUVIX - Infinite Health
FULLCLIP - Infinite Ammo
CVWKXAM - Infinite Oxygen
HESOYAM - Health, Armor, $250K
LXGIWYL - Weapons 1, Grunt
PROFESSIONALSKIT - Weapons 2, Surgeon
UZUMYMW - Weapons 3, Berserker
PROFESSIONALKILLER - Hitman Stat Level
STATEOFEMERGENCY - Chaos Mode
MUNASEF - Adrenal Mode
IOJUFZN - Riot Mode
JCNRUAD - Smash N' Boom
ONLYHOMIESALLOWED - Gang Members Everywhere
FOOOXFT - Armed Peds
BGLUAWML - Peds Attack (Rockets)
AJLOJYQY - Peds Attack Each Other
BAGOWPG - Your are Hunted
SJMAHPE - Recruit Anyone (9mm)
ROCKETMAYHEM - Recruit Anyone (Rockets)
BIFBUZZ - Gangs Rule the Streets
ROCKETMAN - Jetpack
AIYPWZQP - Parachute
BUFFMEUP - Muscle Maxed
BTCDBCB - Fatty
KVGYZQK - Skinny
HELLOLADIES - Sexiness Maxed
BEKKNQV - Slut Magnet
WORSHIPME - Respect Maxed
cvwkxam - Lung Capacity Maxed
NATURALTALENT - Vehicle Skills Maxed
BRINGITON - Wanted Level Maxed
GOODBYECRUELWORLD - Suicide
aezakmi - Wanted Level Never Increases
OSRBLHH - Wanted Level + 2
ASNAEB - Wanted Level Cleared
BLUESUEDESHOES - Elvis has entered the building
YSOHNUL - Fast Clock
SPEEDITUP - Faster gameplay
SLOWITDOWN - Slower gameplay
CJPHONEHOME - Giant BMX Hop
KANGAROO - Super Jump
CIKGCGX - Beach Rave
PRIEBJ - Funhouse Theme
bekknqv - Kinky Theme
CRAZYTOWN - Carnival Theme
BMTPWHR - Trucker Theme
bmtpwhr - Rural Theme
NINJATOWN - Ninja Theme
NIGHTPROWLER - Eternal Midnight
xjvsnaj - 00:00 - 12:00
OFVIAC - 21:00
CWJXUOC - Sandstorm
PLEASANTLYWARM - Sunny Weather
TOODAMNHOT - Super Sunny Weather
SCOTTISHSUMMER - Thunderstorm
AUIFRVQS - Rainy Weather
CFVFGMJ - Foggy Weather
ALNSFMZO - Cloudy Weather
WHEELSONLYPLEASE - Invisible Car
STICKLIKEGLUE - Awesome Handling
YLTEICZ - Aggressive Traffic
GHOSTTOWN - Reduced Traffic
EVERYONEISRICH - Sweet Ride Car Traffic
EVERYONEISPOOR - P.O.S. Car Traffic
FVTMNBZ - Country Traffic
LLQPFBN - Pink Traffic
IOWDLAC - Black Traffic
ZEIIVG - Green Light Traffic
SPEEDFREAK - All Cars Nitrous Powered
BUBBLECARS - Float float away on impact
FLYINGFISH - Flying Boats
RIPAZHA - Flying Cars
CPKTNWT - Cars Explode
VKYPQCF - Nitrous Taxi's, L3 Hop
OHDUDE - Spawn Hunter
MONSTERMASH - Spawn Monster Ride
JUMPJET - Spawn Hydra
VROCKPOKEY - Spawn Racecar
ITSALLBULL - Spawn Dozer
CELEBRITYSTATUS - Spawn Limo
aiwprton - Spawn Rhino
WHERESTHEFUNERAL - Spawn Romero
FOURWHEELFUN - Spawn Quad
TRUEGRIME - Spawn Garbage Truck
FLYINGTOSTUNT - Spawn Stunt Plane
RZHSUEW - Spawn Cadillac
CQZIJMB - Spawn Bloodring Banger
JQNTDMH - Spawn Rancher
KGGGDKP - Spawn Hovercraft
AMOMHRER - Spawn Tanker Truck

Free Car Fix:
Park your car in a garage and back away to let the door come down and close. Then go to the garage so that the door opens again. Use this cheat when you have already pimped out your ride to a good extent.

Two Cars in Garage:
Simply drive one car into the garage so that the door opens, but keep it only halfway in, keeping it open. Now are are free to drive any other car into the garage.