The Simpsons: Hit & Run Cheats

Use the phone booth to call up certain cars in certain levels to ride along with special drivers:

70's Sports Car (Homer) - Levels 3-4
Bandit (Snake) - Levels 1, 3, 4 and 6
Canyonero (Marge) - 2 and 5-7
Electaraus (Lisa) - Levels 4-6
Family Sedan (Homer) - Levels 3-4
Ferrini - Red (Bart) - Level 7
Honor Roller (Bart) - Level 7
Hover Car (Professor Frink) - Levels 1 and 5
Kremlin (Comic Book Guy) - Levels 1, 4 and 5
Longhorn (Apu) - Level 7
Malibu Stacy Car (Lisa) - Levels 4-6
Mr. Burns' Limo (Smithers) - Levels 2 and 4-7
Pickup Truck (Cletus) - Levels 1, 5, and 6
Police Car (Police Chief Wiggim) - Level 1
School Bus (Otto) - Levels 1, 4 and 7
Sedan (Principal Skinner and his mother) - Levels 3 and 5
World War II Vehicle (Grandpa) - Levels 2 and 6
Zombie Car (Living Impaired Individual) - Levels 1-6

Play the game on Halloween or Christmas. The Simpson's home will be decorated accordingly.
You can also change the date in your Playstation and it will appear.