Rockstar Barrels
Look for the 60 yellow barrels throughout the game that have the "Rockstar" logo on them. Some are hidden, while others are in plain sight. Every 10 barrels you collect will open a cheat. After 10 barrels, you'll have unlimited nitro. After 20 barrels, you'll never damage out. When you collect 30 barrels, you get unlimited special abilities. After 40 barrels, you get the top-down view. After 50 barrels, you'll get no police, and after 60 barrels, you'll get no fines. However, using the cheats will prevent you from getting cash and reputation points and hinder you from progressing further into the game.
Karol Garage Partner Mission
Get 33,500 reputation points and achieve "Idol" rank to unlock a special mission called the Karol Garage partner mission. Remember, though, if you have any of your cheats on, you won't be able to get reputation points.
Easy Money
Go into arcade mode, set the AI to easy and traffic to none, and turn off power-ups. As long as you don't come in last place, you'll get some easy money.
No Cops Glitch
Finish a race, and at the end, enable your "No Police" cheat. After you've enabled it, go back and turn it off. The cops will disappear, and you can still gain money and reputation points afterward.
Customize Your Cars
Racing a certain type of vehicle class a specified amount of times will unlock different parts of the car that you can customize. For instance, you can change the blowers on a luxury car or muscle car if you race that vehicle 45 times. Race a vehicle nine times to unlock customizable front bumpers and skirts. Race a vehicle 18 times to unlock customizable rear bumpers, spoilers and motorcycle tails. Race a vehicle 27 times to unlock customizable motorcycle tanks, post gauges, steering wheels, car stereos and front seats. Race a vehicle 36 times to unlock customizable car headlights, hoods, motorcycle bowls and taillights. And finally, race a vehicle 45 times to unlock customizable exhausts, wide body kits and intercoolers.