Citroen Xsara Kit Car:
At the Credits screen, enter Left, Left, A, Right, Right. Or, place first in Mediterranean Rally.

Rally Nissan Micra:
At the Credits screen, enter Left, Up, Left, A, Left. Or, place first in Pro Rally Cup.

Ford RS 200:
At the Credits screen, enter Right, Left, A, Down, Down. Or, place first in all Rallicross Internacional races.

Renault 5 Turbo:
At the Credits screen, enter Down, Right, A, A, Right. Or, place first in all Ice Racing International Circuit races.

Saab 9-3 Viggen:
At the Credits screen, enter Up, Down, Down, Right, A. Or, place first in all Koenig Unlimited races.

Unlock Downhill Tracks:
Enter Geronimo as a driver name.

Unlock Endurance Races:
Enter TheLongHaul as a driver name.

Classic Class Cars:
Enter TheGoodStuff as a driver name to unlock classic cars. Alternately, get over 40,000 points in career mode to unlock the Metro ER4, Peugeot 205 TI6, Audi Quattro SI, and Lancia Delta S4.

Expert Class Cars:
Enter WheelToWheel as a driver name for a few more secret cars.

More stuff:
Earn at least 70,000 points in Career Mode to acquire the Peugeot 405 TI6 Hill Climb, Audi Quattro SI Hill Climb, Suzuki Grand Vitara PP Special, and Toyota Tacoma Hill Climb.

Thanks to Revolution readers Cowboy Bebop Crew, ryan, Ewein, and mike!