Nothing will say that you've unlocked all Free Ride Extreme cars, but if you check the Free Ride car selection screen, the cars should be listed there. Click the right mouse button and the icon will disappear. There is a house with a blue door just to the right of that, go up to the door and the action icon will appear.
Place yourself so that you're looking down the street and look to your right, you should see a telephone booth. The following is a list of PC cheat codes that can be entered during gameplay.
