Enter the following as usernames.

HOTROD Unlock Hot Rod bonus car

FLASH Unlock Phantom bonus car

BIGOVEN All cops speak English

WHIRLY Fly a helicopter

Bonus Cars:
Complete Hot Pursuit mode with each police car to get a new car (BMW M5, Corvette, Porche, Carprice and Lamborghini).

Dashboard:
Select any mode and select a track. Immediately press Up + Triangle + X and hold through the loading screen. This should enable the dashboard.

Turbo Mode:
Once you enable the Dashboard view, hold down the horn to accelerate faster.

Quick money:
You need 2 memory cards and you must own a car to do this trick. Copy your High Stakes file from the first card to the second card. Race in High Stakes mode, pause game play, and have player 2 intentionally forfeit the race. You'll now have two of the same car on the first memory card. Sell 'em for extra money, or keep 'em around as backups!

Drunk mode:
Pick a car and start a race. Immediately press Up + L2 + R1 and hold through the loading screen. The screen will now be blurred...

Easier Tickets:
Start Pursuit Mode and choose a cop car. When the game starts, press L1+Up to turn off the sirens. The person you're chasing will slow down. When you've caught up to him, press L1+Up to turn your sirens back on, then pull him over.

Turn signal/headlights:
Hold L1 and press Left or Right to use the turn signals, or press Down to turn on the headlights.

Super police cars:
To unlock the ability to be the super police cars, you have to arrest 10 speeders within the time limit.

Extra time:
Don't enter a name to get more time in one player Hot Pursuit.