Unlock half of the cars- /6Kb /xMi irLM +/22
Unlock high speed mode- 2QjD +D1Q 9Fy2 5B0C
Unlock everything- S/PD W1/H L8Q8 >VbB

Extra Modes:

Unlock extra mode 1 - at the start screen hold R + L, then press Right + B until a chime is heard.

Unlock extra mode 2 - at the start screen hold R + L, then press Left + B until a chime is heard.

Codes thanks to GameShark.com