Unlimited Turbo & Stuff:
For unlimited turbo/shield, at the menu screen press L1 + R1 then L2 + R2 then L1 + L2 and finally R1 + R2...You will see a message that confirms the code. Must be entered before each race.

Unlimited Ammunition:
Press L2, R2, L1, R1, L2 + R2, L1 + R1 at the team selection screen or the screen with the League Race and XG Mall options. A message will confirm correct code entry. Unlimited ammunition will be available in one or two player games. This is especially useful when paired with the Rear Blaster item.
Information in this section was contributed by Jon Hatley.

All Tracks:
Press L1, L1, L2, L2, R2, R2, R1, R1, L1 + R1 + L2 + R2 at the main menu. A message will be heard for correct code entry.

Always Win XG Career Mode Races:
Press R2 + L2 + R1 + L1, R2 + L2, R1 + R2, R2 + R1 + L2 + L1 at the main menu. A message will be heard for correct code entry. Permits you to win a race no matter if you quit, lose, or die. Note: This must be enabled at the main menu before every race.

Double Prize Money:
Press L1 + L2, R2 + R1, R1 + R2, L2 + L1 at the main menu. A message will be heard for correct code entry.

Extreme Lap Challenge:
Press L1, L2, L1, R1, L1, R2, L1 + R1, L2 + R2 at the main menu. A message will be heard for correct code entry.

StarCom Team:
Press L1, L2, R2, R1, R1, R2, L2, L1 at the main menu. A message will be heard for correct code entry.

Thanks to Revolution reader BudSmoker420!