You go to your phone (in-game) and dial the following numbers.Be careful as some of these cheats will block your achievements.
After you have dialed the desired cheat it should say "Cheat Activated" then go to the Cheats menu in your (in-game) phone and select the cheat you want.

Spawn Cars
Any time you're outside, break out your cell phone and insert the following phone numbers to unlock the vehicles listed.

Annihilator (Helicopter) - (359)-555-0100
Cognoscenti (Car) - (227)-555-0142
Comet (Car) - (227)-555-0175
FIB Buffalo (Car) - (227)-555-0100
Jetmax (Boat) - (938)-555-0100
NRG-900 (Motorcycle) - (625)-555-0100
Sanchez (Car) - (625)-555-0150
Super GT (Car) - (227)-555-0168
Turismo (Car) - (227)-555-0147

Change Weather & Time of Day
To randomly change the weather and time of day, break out your cell phone and insert the following number: (468)-555-0100.

Weapon Batch 1
To unlock a baseball bat, grenades, M4, MP5, pistol, RPG and sniper rifle, insert the following number into your cell phone: (486)-555-0100. Keep in mind that this will disable your ability to acquire some achievements.

All Secret Locations
Head to the internet caf (or wherever you can access the internet) and insert the URL [ Register or Signin to view external links. ] . If inserted properly, you'll be brought to an in-game message board containing maps of secret locations in the game. Check it out!

Weapon Batch 2
To unlock an AK-47, knife, Molotov Cocktails, pistol, RPG, sniper rifle and Uzi, insert the following number into your cell phone: (486)-555-0150. Keep in mind that this will disable your ability to acquire some achievements.

Adjust Your Wanted Level
To get rid of Niko's wanted level, insert the following number into your cell phone: (267)-555-0100. To add stars to Niko's wanted level, insert this number instead: (267)-555-0150. Keep in mind that this will disable your ability to acquire some achievements.

Refill Health, Armor & Ammo
To refill Niko's health and to give him some fresh armor, insert the following number into your cell phone: (362)-555-0100. If you want to give him health, fresh armor and refill all ammunition, insert this number instead: (482)-555-0100. Keep in mind that this will disable your ability to acquire some achievements.

Unlockables:
GTA3 Outfit
At one point in the game, you're given an option to kill Dwayne or Playboy X, and you must choose one or the other. If you kill Playboy X, you can use his loft and change clothes there. Amongst your change of clothes will be the outfit from GTA3 -- a tight black jacket and green cargo pants.

Statue of Happiness Shirt
Head over to the Statue of Happiness and climb up as far as you can go. Head around the left side of the statue from where you entered. When you do this, the game will pause briefly and load a new shirt that your character will be wearing, a nice souvenir for visiting the statue!
Hints:
Losing the Cops
No matter the extent of your wanted level, if you can make it back to one of your safe houses and go to your bed to save, the cops will be automatically called off. You dont even have to actually save you can just access the save menu and then cancel out. As long as you reach this menu, then you wont have to worry about the cops chasing you once you go back outside.

Friendship Perks
To unlock the perks of friendship with any given person below, reach the friendship level listed.

Brucie - Brucie likes to bowl, drink, eat, go to strip clubs, shows, helirides and boating. If you get his friendship level up to 70%, you can call Brucie and he'll dispatch a helicopter to your location, allowing you to utilize it as you'd like.

Dwayne - Dwayne likes to go to strip clubs, eat, drink, go to shows and bowl. If you get his friendship level up to 60%, you can call Dwayne and he'll send you a car full of gang members that will follow you and help you out as you need them.

Little Jacob - Little Jacob likes to play darts, go to shows, play billiards, drink, eat, and go to strip clubs. If you get his friendship level up to 70%, you can call Little Jacob and he'll drive a car to your location full of guns that you can purchase from him.

Packie - Packie likes to go to strip clubs and shows, play darts, drink, bowl and play billiards. If you get his friendship level up to 75%, you can call Packie and he'll make you a potent car bomb that you can use as you need it.

Roman - Roman likes to go bowling, play darts, go drinking, eat, play billiards, go to shows and to strip clubs. If you get his friendship level up to 60%, you can call Roman and he'll send you a free cab to use anywhere in the city.

Identify Unknown Songs
If a song is playing on your radio and you want to identify the artist and song title, insert (948)-555-0100 in your phone. You'll receive a text message with the information you seek.

Free Taxi Rides
Here's a great way to get a free taxi fare. Hail a taxi and get inside, and head to your destination. Right before you get to your destination, bust out the back window and point your gun out of the window (using the corresponding controls). Doing so will freak the taxi driver out. He'll stop the car and run away, leaving you near your destination without forcing you to pay for the ride.

Restart A Dead Car
If your car gets horribly damaged and stalls or refuses to start, call 911 on your cell phone. When they pick up, hang up your phone, and your car should start once more!

Easy Money
Head to an ATM anywhere in the city, and cause a traffic jam in the area. Wait for someone to approach the ATM and take money out of it, and then eliminate that person. The traffic jam should ensure that no ambulance can reach the injured person, so if you run a short distance away and return, you can grab the money that dead person dropped over and over again.

Long-Lasting Fire Truck
Fire trucks are awesome vehicles to use to cause complete and utter destruction in Liberty City. But, even these vehicles are susceptible to damage. Oddly, if the fire truck catches on fire and is about to explode, you can actually utilize the fire truck's very own water source to put its own fire out. Nice!

100% Completion
Below is a list of how to reach 100% completion, and what everything in the game is worth (by percentage).

Ninety Story/Procedural Missions - 68%
Acquire each friend's special ability and do their activities - 10%
Win at each activity and mini-game - 5%
Find all random characters and complete their missions - 5%
Kill all thirty wanted persons - 2.5%
Complete all twenty vigilante crimes - 2.5%
Complete all fifty stunt jumps - 2.5%
Eliminate all two-hundred pigeons - 2.5%
Complete all thirty car thefts - 2%

Pigeon Locations
The two maps below represent the location of all of the pigeons in the game.



Stunt Jump Locations
The map below represents the location of all of the stunt jumps in the game.
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Armor Locations
The map below represents the location of all of the armor in the game.
[ Register or Signin to view external links. ]

Easter Eggs:
Fire Truck Madness
If you boost a fire truck, press the fire button while driving around. This will allow you to shoot the super-powerful water cannon at anyone around you. Watching people react to getting shot with high-speed water is hilarious.

Statue of Happiness
Head to the Statue of Happiness on Happiness Island via helicopter. Have the helicopter drop you off on the very top of the statue. You'll find a sign up there that states "No Hidden Content Beyond This Point", but it's a lie. If you head inside of the statue from this area, you'll find an actual beating heart inside the statue. How ridiculously creepy!

Prank Calls
Here's something funny to try. During your adventure through Liberty City, you may find phone numbers written in inconspicuous places, such as on the side of a building or in the subway. If you dial these numbers, you'll usually get someone on the other end who you can subsequently prank.

Websites:
bigdog.blogsnobs.org
darksoullock.blogsnobs.org
eddielowfilthslayer.blogsnobs.org
fefighter2008.blogsnobs.org
inyapuper.blogsnobs.org
liberatedwoman.blogsnobs.org
limevenus.blogsnobs.org
mail.eyefind.info
pychakilla.blogsnobs.org
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[ Register or Signin to view external links. ] *INSTANT 5 STAR WANTED LEVEL*
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****
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[ Register or Signin to view external links. ] for the internet cafe TW@
Phone numbers:

Bean Machine Coffee: 555-0110
Emergency Services: 911
Express Car Service: 555-2222
Express Car Service: 555-3333
Fire Proof Doors: 555-5700
Hero Shop: 555-8575
LCPD Recruitment: 555-RECRUIT
Liberty Construction: 555-1274-73
No Problemo Bail Bonds: 555-945-733-92
Storage: 555-221-3877
Superb Deli: 555-4674
The Serrated Edge: 1-800-555-5555
Used Auto Parts: 555-7300
Vinewood Laundromat: 555-5123
Zit Song Identifier: Dial "948-555-0100" - his will identify the song playing on your radio.

Blackmailer from "Call And Collect" mission: 843-555-0124
Drug dealer from "Lure" mission: 545-555-1022
Roman Bellic: 718-926-7215

PC:
Health And Weapons = GTA-555-0100
Weapons = GUN-555-0150
Advanced Weapons = GUN-555-0100
Health = DOC-555-0100
Wanted Level Down = COP-555-0100
Wanted Level Up = COP-555-0150
Climate Change = HOT-555-0100
Spawn Cognoscenti (Mafia Car) = CAR-555-0142
Spawn Sanchez (Dirt Bike) = MBK-555-0150
Spawn FBI Buffalo = CAR-555-0100
Spawn Turismo = CAR-555-0147
Spawn NRG900 = MBK-555-0100
Spawn Comet = CAR-555-0175
Spawn SuperGT = CAR-555-0168
Spawn Police Chopper = FLY-555-0100
Spawn Jetmax (Boat) = WET-555-0100

PS3:
Health And Weapons = GTA-555-0100
Weapons = GUN-555-0150
Advanced Weapons = GUN-555-0100
Health = DOC-555-0100
Wanted Level Down = COP-555-0100
Wanted Level Up = COP-555-0150
Climate Change = HOT-555-0100
Spawn Cognoscenti (Mafia Car) = CAR-555-0142
Spawn Sanchez (Dirt Bike) = MBK-555-0150
Spawn FBI Buffalo = CAR-555-0100
Spawn Turismo = CAR-555-0147
Spawn NRG900 = MBK-555-0100
Spawn Comet = CAR-555-0175
Spawn SuperGT = CAR-555-0168
Spawn Police Chopper = FLY-555-0100
Spawn Jetmax (Boat) = WET-555-0100


The Lost and Damned
Xbox360:
Restore/Give Armor = 362-555-0100
Restore Armor/Health/Ammo = 482-555-0100
Lower Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Burrito = 826-555-0150
Spawn Double T = 245-555-0125
Spawn Hakuchou = 245-555-0199
Spawn Hexer = 245-555-0150
Spawn Innovation = 245-555-0100
Spawn Slamvan = 826-555-0100

PC:
Restore/Give Armor = 362-555-0100
Restore Armor/Health/Ammo = 482-555-0100
Lower Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Burrito = 826-555-0150
Spawn Double T = 245-555-0125
Spawn Hakuchou = 245-555-0199
Spawn Hexer = 245-555-0150
Spawn Innovation = 245-555-0100
Spawn Slamvan = 826-555-0100

PS3:
Restore/Give Armor = 362-555-0100
Restore Armor/Health/Ammo = 482-555-0100
Lower Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Burrito = 826-555-0150
Spawn Double T = 245-555-0125
Spawn Hakuchou = 245-555-0199
Spawn Hexer = 245-555-0150
Spawn Innovation = 245-555-0100
Spawn Slamvan = 826-555-0100

The Ballad of Gay Tony
Xbox360:
Restore Health/Armor = 362-555-0100
Restore Health/Ammo = 482-555-0100
Remove Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Change Weather = 468-555-0100
Give Parachute = 359-555-7272
Explosive Punch = 276-555-2666
Explosive Sniper Rounds = 486-555-2526
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Floater = 938-555-0150
Spawn Akuma = 625-555-0200
Spawn Vader = 625-555-3273
Spawn APC = 272-555-8265
Spawn Buzzard = 359-555-2899
Spawn Bullet GT = 227-555-9666
Spawn Annihilator = 359-555-0100
Spawn Jetmax = 938-555-0100
Spawn NRG-900 = 625-555-0100
Spawn Sanchez = 625-555-0150
Spawn FIB Buffalo = 227-555-0100
Spawn Comet = 227-555-0175
Spawn Turismo = 227-555-0147
Spawn Cognoscenti = 227-555-0142
Spawn Super GT = 227-555-0168

PC:
Restore Health/Armor = 362-555-0100
Restore Health/Ammo = 482-555-0100
Remove Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Change Weather = 468-555-0100
Give Parachute = 359-555-7272
Explosive Punch = 276-555-2666
Explosive Sniper Rounds = 486-555-2526
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Floater = 938-555-0150
Spawn Akuma = 625-555-0200
Spawn Vader = 625-555-3273
Spawn APC = 272-555-8265
Spawn Buzzard = 359-555-2899
Spawn Bullet GT = 227-555-9666
Spawn Annihilator = 359-555-0100
Spawn Jetmax = 938-555-0100
Spawn NRG-900 = 625-555-0100
Spawn Sanchez = 625-555-0150
Spawn FIB Buffalo = 227-555-0100
Spawn Comet = 227-555-0175
Spawn Turismo = 227-555-0147
Spawn Cognoscenti = 227-555-0142
Spawn Super GT = 227-555-0168

PS3:
Restore Health/Armor = 362-555-0100
Restore Health/Ammo = 482-555-0100
Remove Wanted Level = 267-555-0100
Raise Wanted Level = 267-555-0150
Change Weather = 468-555-0100
Give Parachute = 359-555-7272
Explosive Punch = 276-555-2666
Explosive Sniper Rounds = 486-555-2526
Give Weapon Package v1 = 486-555-0150
Give Weapon Package v2 = 486-555-0100
Spawn Floater = 938-555-0150
Spawn Akuma = 625-555-0200
Spawn Vader = 625-555-3273
Spawn APC = 272-555-8265
Spawn Buzzard = 359-555-2899
Spawn Bullet GT = 227-555-9666
Spawn Annihilator = 359-555-0100
Spawn Jetmax = 938-555-0100
Spawn NRG-900 = 625-555-0100
Spawn Sanchez = 625-555-0150
Spawn FIB Buffalo = 227-555-0100
Spawn Comet = 227-555-0175
Spawn Turismo = 227-555-0147
Spawn Cognoscenti = 227-555-0142
Spawn Super GT = 227-555-0168
Let me know if there is anything I should add and if there is anyway for me to make the thread cleaner!