Super speed:
At the Options menu, enter JIH345 as your password. To disable, you must reset your PS2.

Ride a Slipknot 'Cycle:
At the Options menu, enter 86657457 as your password.

Slipknot Video:
At the Options menu, enter 23F7IC5 as your password.

Speed Demon mode:
Beat the MX Career mode.

Trickbot mode:
Beat the Freestyle mode.

Ride as Officer Dick:
Beat all the Freestyle competitions.

Thanks to Revolution readers Jimmy tricky, fred86, chills, Donny and PeePerbottle!