Enter any of the following cheat codes through the Settings > Cheats menu. Most of these can then be toggled on/off.

Code - Effect

TOOLEDUP - Free Garage
IRONMAN - God Mode
ROLLBAR - Indestructible Cars
ZOOMZOOM - Unlimited Nitro
GUNBELT - Unlimted Ammo
CARSHOW - Unlock All Vehicles
GUNRANGE - Unlock All Weapons
KEYSTONE - Weak Cops