Cheat mode

While playing the game, press Up to display Niko's cell phone. Press Up again to access the keypad. Then, enter one of the following codes to activate the corresponding cheat function. Once a code has been entered correctly, a new "Cheats" menu option will be available on your phone, below "Options", where you can access the cheats without having to enter the phone number again. Note: Do not save the game after enabling a code to avoid unforeseen problems. Enabling some cheats will prevent the indicated achievement(s) from being earned. You can usually only spawn one vehicle at a time. Any previously spawned vehicles will disappear. However, if you spawn a vehicle, save the game, and load again, you can spawn another one and your original will not disappear. You must have it in your parking space in front of a safehouse or it will be gone before you have a chance to spawn another.

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". This code prevents the "Cleaned The Mean Streets", "Finish Him", "One Man Army", and "Walk Free" achievements from being earned.
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". This code prevents the "Cleaned The Mean Streets" achievement from being earned.
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". This code prevents the "Cleaned The Mean Streets" achievement from being earned.
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". This code prevents the "Cleaned The Mean Streets" achievement from being earned.
Remove Niko's wanted level

Dial "2675550100" on the cell phone. Note: This phone number translates to "COP-555-0100". This code prevents the "One Man Army" and "Walk Free" achievements from being earned.
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". This code prevents the "One Man Army" and "Walk Free" achievements from being earned.
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".

The following codes require "The Lost And Damned" bonus downloadable content:

Spawn Burrito

Dial "8265550150" on the cell phone.
Spawn Double T bike

Dial "2455550125" on the cell phone.
Spawn Hakuchou bike

Dial "2455550199" on the cell phone.
Spawn Hexer bike

Dial "2455550150" on the cell phone.
Spawn Innovation bike

Dial "2455550100" on the cell phone.
Spawn Slamvan

Dial "8265550100" on the cell phone.

I wish they had airplanes in that game

Does anyone honestly play GTA anymore. I thought it died when COD4 came out.