Cheats:
Input the following on the cheats menu.

GUNBELT - Infinite ammunition.
GUNRANGE - Unlock all weapons.
ZOOMZOOM - Infinite nitro.
ROLLBAR - Cars are indestructable.
TOOLEDUP - Upgrades cost nothing.
CARSHOW - Unlocks all vehicles.
IRONMAN - Player invulnerability.
KEYSTONE - Police vehicles vulnerable.

Unlockables:
Double Ammunition Capacity - You must collect 30 Stars
Double Car Durability - You must collect 40 Stars
Free Vehicle Upgrades - You must collect 50 Stars
Health Increase - You must collect 10 Stars
Nitrous Increase - You must collect 20 Stars
 
Atlus Racer, Negotiator Vehicle, Ram Raider, Then & Now Change Option:
Finish the Game.

Unlockable: F70 Weapon
Finish the mission "Tailgate".

Unlockable: Gangster Weapon
Finish the mission "Tailgate".

Unlockable: Service 9 Weapon
Finish the mission "Last Chance".

Unlockable: L15 Weapon
Finish the mission "Jail Break".

Unlockable Odometer Modes:
Bodysnatcher - Accumulate 666 miles on the Odometer
Night Night - Accumulate 700 Miles on the Odometer
Shortest Day - Accumulate 800 Miles on the Odometer
Far Out - Accumulate 900 Miles on the Odometer

Unlockable: Free Garage
The player can also unlock the free garage by maxing out his cash counter ,999. Once this is filled it will unlock the free garage

Unlockable: TOKENS
Collecting certain numbers of tokens unlocks permanent additions to the game that enhance the player's abilities, e.g. double health, double nitro, double ammo, etc. Once collected they don't go away within the current game.

Then
10 tokens: +50 Health. 10/50 tokens collected. 50% more health capacity.
20 tokens: Double nitro (length of nitro doubled). 20/50 tokens collected. Nitro's doubled.
30 tokens: Double ammo capacity - All guns. 30/50 tokens collected. Ammo capacity doubled.
40 tokens: Double cars toughness (Only cars in v-edit). 40/50 tokens collected. Saved cars armour plated.
50 tokens: Free vehicle mod's. 50/50 tokens collected. Free vehicle mod's.

Now
10 tokens: +50 Health. 10/50 tokens collected. +50% Health.
20 tokens: Double nitro. 20/50 tokens collected. Nitro's doubled.
30 tokens: Double ammo capacity - All guns 30/50 tokens collected. Ammo capacity doubled.
40 tokens: Double cars toughness. 40/50 tokens collected. Saved cars armour plated.
50 tokens: Free vehicle mod's. 50/50 tokens collected. Free vehicle mod's.

All of the above cheats reset after the era change.

Once the player has collected the relevant amount of tokens to unlock a cheat a text message is displayed to inform the player that it has been unlocked. This is displayed a few seconds after the player has collected the final token to allow time for the thrill-cam to have stopped.

Unlockable: Hot Rod
You must come in 1st in Driver GP Long Island Race, during the 2006 era.