Cell Phone Passwords

At any time during the game, pull out Niko's phone and dial these numbers for the desired effect.

468-555-0100 - Change weather
486-555-0150 - Get a different selection of weapons
486-555-0100 - Get a selection of weapons
267-555-0150 - Raise wanted level
267-555-0100 - Remove wanted level
362-555-0100 - Restore armour
482-555-0100 - Restore health, armor, and ammo
948-555-0100 - Song information
227-555-0142 - Spawn a Cognoscenti
227-555-0175 - Spawn a Comet
938-555-0100 - Spawn a Jetmax
625-555-0150 - Spawn a Sanchez
227-555-0168 - Spawn a SuperGT
227-555-0147 - Spawn a Turismo
359-555-0100 - Spawn an Annihiliator
227-555-0100 - Spawn an FIB Buffalo
625-555-0100 - Spawn an NRG-900

Map Locations

Enter the following password into the in-game computers.

http://www.whattheydonotwantyoutoknow.com - Weapon, health, armor, vehicle, pigeon, stunt, and entertainment locations

Friendship Bonuses

By gaining friendship with the following people can benefit you in many ways.

50% off for all Clothing Stores - Get 80% Relationship Status with Alex
Boom? (Call Packie for him to make you a car bomb) - Gain 75% friendship with Packie
Chopper Ride (He will pick you up in his helicopter) - Gain 70% Friendship with Brucie
Discount Guns (Buy weapons at a cheaper price from Lil Jacob) - Gain 60% friendship with Little Jacob
Extra Help (A car of gang members will be sent to help you out) - Gain 60% friendship with Dwayne
Free Ride (Call for a taxi) - Gain 60% friendship with Roman
Health Boost (Call Carmen and select "Health Boost") - Get 80% Relationship Status with Carmen
Remove Up to 3 Wanted Stars (Call Kiki and select "Remove Wanted") - Get 80% Relationship Status with Kiki

Unlockables

Annihilator Helicopter - Kill all 200 Flying Rats
Rastah Color Huntley SUV - Complete 10 Package Delivery missions
Remove Ammo Limit - Get 100% completion

Secrets

Easy money

Go to an ATM, then cause a traffic jam to the area. Wait for someone to take money from the ATM, then kill him or her. Make sure to block any nearby roads so the ambulance cannot get to the murdered victim. Take the money they dropped on the ground. Run a short distance away, and go back. The money should have respawned on the ground. You can repeat this as many times as desired. You can also kill more people taking their money from the ATM to increase the amount of money that respawns.

The Heart of Liberty City

First, you'll have to have access to Happiness Island. Once you're able to go there legally, find the Helicopter Tours (which is directly east of Happiness Island) and steal a helicopter. Fly to Happiness Island and over the Statue of Happiness, then jump out of the helicopter at the statue's feet. You should land on the topmost tier of the statue, which is basically a square platform with a door in the center of each side. Run around the platform until you see a door with a plaque on either side of it that reads, "No Hidden Content Here." It may seem like you cannot go through the door, but you can-- it doesn't open, you simply walk THROUGH the door. Inside, you'll find an empty room with a tall ladder. Climb it, and when you reach the top, look up; there is a gigantic beating heart, held in place by chains.

Get a Statue of Happiness T-shirt

Go to the Statue of Happiness and walk up to the second level. Walk through the door (it doesn't swing open, just go through it) and the game will load for a second and you will walk out with a new t-shirt with the Statue of Happiness on it.

Repairing the engine

If the vehicle you are driving breaks down and the engine will not start, call any number from your phone book, and your engine should start again.

Source