Unlockables:
Enter any of the following codes at the DNA lab to unlock a challenge. Win that challenge to unlock the corresponding cars at the DNA lab:

Ascari KZ1 - KNOX
Audi TT 1.8L Quattro - YTHZ
BMW Z4 Roadster - GVDL
Frito-Lay Infiniti G35 - MNCH
Holden Monaro - RBSG
Hyundai Coupe 2.7L V6 - BSLU
Infiniti G35 - MRHC
Infiniti G35 (red design) - MNCH
Koenigsegg CCX - KDTR
Mitsubishi Prototype X - DOPX
Nissan 350Z - PRGN
Nissan Skyline R34 GT-R - JWRS
Saleen S7 - WIKF
Seat Leon Cupra R - FAMQ

Celebrity race challenge unlockable cars:
Unlock the following cars by beating the corresponding celebrities in all 3 of his race challenges:

Audi 1.8T Quattro - Beat caballero in all 3 of his race challenges.
Dodge Viper SRT-10 - Beat samuel hubinette in all 3 of his race challenges.
Ford Mustang Fastback - Beat djibril cisse in all 3 of his race challenges.
Ford Mustang GT '05 - Beat smudo in all 3 of his race challenges.
Nissan Silvia S15 Spec-R - Beat phil morrison in all 3 of his race challenges.
Nissan Skyline R32 GTS-t Type M - Beat tarzan yamada in all 3 of his race challenges.
Toyota Corolla GT-S - Beat beau yates in all 3 of his race challenges.

Mazda RX-8:
The Mazda RX-8 is the fastest in stock.

Easy money:
To earn easy money, go to World Class League and spectate Air Trauma. Bet on Crystal, who's odds are 20:1. At the very beginning of the race, cheer(R1) for Crystal to give her a full nitrous bar. Boo(L1) any cars that are in front of her and have gained nitrous. Continue to do this until she has passed everyone.

Last Man Standing challenge tip:
Get in last place and spook each car one at a time. It may take more than one attempt, but when done successfully the car will spin out and hit the wall, eliminating it from the race.