Press Shift + ~ (tilde key - usually below the escape key) and enter the following codes:

BADGANGSTERS - Godmode
DEADGUNS - Guns and Ammo

Invisible Car

In the construction site near Hoboken, drive around the site with your car passing buildings until you see a car on your radar. To get to the car, walk through the big wooden door (yes THROUGH it) and drive back out. After about 30 seconds the car starts disappearing. When the car is done disappearing, YOU will too, and if you save, then the effect will become permanent.

Hints:

Get Silver Fletcher (easier method)

On the mission 'Visiting rich people' there is an alternative way to get the Silver Fletcher (this is the so called bullet proof tank of a car Morello drives). When you get to the final part of the mission when you are leaving the villa, you should see the car the owner of the house pulled up in under a car port. Walk up to it and then ask that guy that's following you to open it up as you did getting into the villa in the first place. He will lock pick it and you would have learned how to lockpick it. Presto! You now have the Silver Fletcher and don't have to spend ages trying to get it from the airport on mission 16 or something when you're trying to assasinate Morello.

Unlock Free Ride Extreme mode

To unlock the Free Ride Extreme you have to finish all missions. The Extreme mode allows you to get extra fancy cars (like the super fast taxi). Also there are no cops in this mode.

Easy money

1. Kill the men in the all black suits with the black hat (gangsters) to earn $500 each.
2. Go into the country and speed in a fast car.
3. Blow up cars for $100 each.
4. Drive taxis to earn money.