Enter the options screen, select the load/save option, and go to the password screen. Now enter these passwords to get these:

Sergei:
Square, L1, Square, L2, Triangle, R2, X, Circle

Jackal:
Enter L2, L2, Circle, R2, Square, R1, L1, Triangle

Rhine Rider:
Enter Circle, Triangle, Square, R2, R2, L1, X, Triangle

Star Cay:
Enter Circle, Triangle, Square, R2, R2, L1, X, Triangle

Mirrored tracks:
Enter Circle, L1, R2, R2, R1, X, Triangle, L2

Fog mode:
Enter Square, L1, X, Square, R2, X, Triangle, Circle

Cheaper upgrades:
Enter X, L1, Circle, Triangle, Square, Square, Triangle, L2

Dance mix:
Enter R2, Triangle, X, R2, Triangle, Square, Circle, X

Super Snocross 6:
Enter Square, X, Triangle, Triangle, L1, R2, Circle, Triangle

Demo track:
Enter R2, L1, Triangle, Square, Triangle, R1, Circle, X

Open Mountain champion:
Enter Square, X, R2, Square, Circle, R1,Circle, Triangle