GTA IV cheats and secrets

If your car engine fails but the car hasn't started burning then call anyone on your phone and the engine will start up again.

You can stop your car instantly by using the camera function on your phone.

There are many phone numbers in the game but I only know of two that actually answer.

911 - Emergency Services - order a cop car, ambulance or firetruck
948-555-0100 - ZiT - displays the name of the song playing on the radio

To view location maps of pigeons, weapons, armour, stunt ramps, health, entertainment and vehicles, use one of the in-game computers and type: www.whattheydonotwantyoutoknow.com

Cheats

Some cheats block XBOX 360 achievements.  They tell you which achievement they block after you enter them.

I don't recommend using cheats until after you have finished all the missions because it removes the challenge and could have some unwanted side effect. I recommend disabling autosave in the game options before using cheats so you don't accidentally save with a cheat enabled (it gets recorded in your save statistics).  Cheats are entered using Niko's cell phone (push d-pad up twice, enter the numbers and then select call. A cheat menu is added to your phone after you enter a cheat, so you can apply the cheat again without having to enter the number (just select it in the cheat menu).  If you want to save after using a cheat then use a new save slot. It's always a good idea to have multiple saves.