GTA 4:

Full health and armor
Dial "3625550100" on the cell phone. If you enter this code while in a vehicle, it will also repair it. Note: This phone number translates to "DOC-555-0100".

Full health, armor, and ammunition
Dial "4825550100" on the cell phone. If you enter this code while in a vehicle, it will also repair it. Note: This phone number translates to "GTA-555-0100".

Weapons tier 1
Dial "4865550100" on the cell phone. This will unlock the baseball bat, handgun, shotgun, MP5, M4, sniper rifle, RPG, and grenades. Note: This phone number translates to "GUN-555-0150".

Weapons tier 2
Dial "4865550150" on the cell phone. This will unlock the knife, Molotov cocktails, handgun, shotgun, Uzi, AK47, sniper rifle, and RPG. Note: This phone number translates to "GUN-555-0100".

Remove Niko’s wanted level
Dial "2675550100" on the cell phone. Note: This phone number translates to "COP-555-0100".

Add one star to Niko’s wanted level
Dial "2675550150" on the cell phone. Note: This phone number translates to "COP-555-0150".

Spawn Annihilator police helicopter
Dial "3595550100" on the cell phone. The Annihilator is armed with rockets. Note: This phone number translates to "FLY-555-0100".

Spawn Cognoscenti
Dial "2275550142" on the cell phone. Note: This phone number translates to "CAR-555-0142".

Spawn Comet
Dial "2275550175" on the cell phone. Note: This phone number translates to "CAR-555-0175".

Spawn FBI Buffalo
Dial "2275550100" on the cell phone. Note: This phone number translates to "CAR-555-0100".

Spawn Jetmax
Dial "9385550100" on the cell phone. Note: This phone number translates to "WET-555-0100".

Spawn NRG-900
Dial "6255550100" on the cell phone. Note: This phone number translates to "MBK-555-0100".

Spawn Sanchez
Dial "6255550150" on the cell phone. Note: This phone number translates to "MBK-555-0150".

Spawn SuperGT
Dial "2275550168" on the cell phone. Note: This phone number translates to "CAR-555-0168".

Spawn Turismo
Dial "2275550147" on the cell phone. Note: This phone number translates to "CAR-555-0147".

Change weather and brightness
Dial "4685550100" on the cell phone. Note: This phone number translates to "HOT-555-0100".