Here are a heap of cheats for Midnight Club 3: DUB Edition Remix. For all of the following enter the cheat codes at the cheat menu to unlock the cheat that you want.

In all cases, they cheat menu is here :: Main Menu >> Options >> Cheats

Unlock all cities in arcade mode
Roadtrip

Increase car mass in arcade mode
Hyperagro

Unlock Argo special move
Dfens

Unlock Roar special move
Rjnr

Unlock Zone special move
Allin

No damage
Ontheroad

Faster pedestrians
Urbansprawl

And here are some of the funny ones...

Get a bunny head
Getheadl

Get a flaming head
Trythisathome

Get a pumpkin head
Getheadk

Get a skull head
Getheadn

Get a snowman head
Getheadm

Get a yellow smile head
Getheadj

Get a chrome body
Haveyouseenthisboy

Get more money
Enter "kubmir" at the cheat menu. Adds one dollar so you'll need some kind of auto fire to amass some bucks quickly... Try this... After you have entered the code, press R2 and set the word "kubmir" to the top slot. Now set R2 and X to auto-fire. Hold down the R2 and X and hopefully the bucks will start to roll in.

Here are some gameplay unlocks if you don't fancy being a cheat...

Chevy Cobalt SS
Successully complete the Unbeatable Street Racers tournament.

Dodge Magnum
Successully complete the Ginza tournament.

Gemballa F355
Successully complete the By Invitation Only tournament.

Hotmatch Chingon
Successully complete the Choppers Of America tournament.

Hummer H3T
Successully complete the Big Playas tournament.

Nissan Sport Concept
Successully complete the Class C Vehicles Only tournament.

Paul Smart Sports Bike
Successully complete the Original Riders tournament.

Tokyo Police Car
Successully complete all tournaments in Tokyo Challenge mode.