Activating Grand Theft Auto 4's cheats is simple, you just have to display Niko’s cell phone and then press up to access the phone's keypad. After that just enter the cheat code into the keypad and you'll have access to a new cheats menu whenever you want. But don't save the game after enabling the cheats as it's reported to cause some bugs.
Among the long list of cheats there's the obligatory 'full health' and 'full ammunition', but it's the 'Spawn Annihilator police helicopter' that looks like the winner of the cheats that have surfaced so far. For a full list of all GTA IV cheat codes just click here.