Midnight Club II

To change the difficulty, go to the Options menu, and select Options: Cheat Codes. Enter the code howhardcanitbe, followed by a number 0-9. The most difficult is howhardcanitbe9, while the easiest is howhardcanitbe0. Enjoy.

Highlight Options on the main menu then highlight Options: (Name of Player). Press LEFT or RIGHT on that selection to access the Cheat Code menu. Press A to start entering cheats, and R to copy/paste cheat to/from the Xbox clipboard.

hotwired - unlocks all cars. Only works while logged into Xbox Live.
theworldismine - unlocks all arcade locations.
zooomzoom4 - inlimited nitro.
greasemonkey - all car abilities
bigbrother - unlocks most dedicated status.
howfastcanitbe0-9 - set game speed (0 is low, 9 is fast)
howhardcanitbe0-9 - set game difficulty (0 is easy, 9 is hard)

While driving the motorcycle, intentionally hit something to throw off the rider. Pause the game before he gets back on the bike. Now unpause and the bike will have no rider!

To activate guns and rockets, enter the cheat "lovenotwar" at the Cheat Codes screen in the Options menu. Fire them by clicking the analog sticks.

To have unlimited nitrous, enter the cheat "zoomzoom4" at the Cheat Codes screen in the Options menu.

To unlock all car abilities, enter "greasemonkey" at the Cheat menu.

To unlock all locations, enter "theworldismine" at the Cheats menu.

To unlock the LAPD car, beat all circuit races in LA.

To unlock the motorcycle, beat five motorcycles in a long race.

Beat all circuit races in Paris (Arcade Mode) to unlock the Parisian police car.

Finish first in the Arcade races in Tokyo, LA and Paris to unlock the SLFA50X, which is only usable in Arcade Mode.

To unlock the Tokyo Police Car, beat all circuit races (Arcade Mode) in Tokyo.