All teams
Press [Triangle], [Left], [Right], [Triangle]x2,
[Left]x2, [Right]x2 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]x2, [Left]x2, [Down]x2,
[Left]x2, [Up]x2, [Right]x2, [Down]x2,
[Right]x2 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 opponent's defense. This was deemed too useful a move
by the developers and thus was not mentioned in the game manual.