All teams:
Press Triangle, Left, Right, Triangle(2), Left(2), Right(2) at the
main menu. This will unlock all teams (all past winners of the UEFA
Champions League since 1960 plus the three hidden developer teams),
and cause a message box to appear as if you had just unlocked the
final past winner (Real Madrid 00). It will also show the credits
sequence the next time you re-enter the frontend having left a
match.
Debug mode:
Press Up(2), Left(2), Down(2), Left(2), Up(2), Right(2), Down(2),
Right(2) in the options menu within a tournament. This will
activate a self-test routine which simulates random keypresses
within the menus.
Hidden move:
During the game while on the ball, press R1 quickly followed by R2.
This will cause your player to perform a pirouette spin, allowing
you to skip past opposing players easily. It is difficult to get
the timing correct, but it really splits your opponents defense.
This was deemed too useful a move by the developers and thus was
not mentioned in the game manual.