Adidas Power Soccer '98

At the Main start-up screen hold L1+L2+R1+R2. This will enable the cheat entry screen. Now enter the following codes...
Big head mode:
Press Square, Circle, Circle, X, Triangle, X, Square, Circle.

Hidden teams:
Press X, Triangle, X, Square, Circle, Square, X, Triangle. A new flag will appear to the right of The Netherlands.

View credits:
Press Circle, Square, Triangle, Circle, X, Triangle, Triangle, Square.

Bonus stadium:
Press Square, Circle, Square, Triangle, Triangle, Circle, X, X

Win match:
Press X, Triangle, Triangle, Circle, X, Square, Circle, Triangle. Then, pause gameplay to win the match.

Stupid commentaries:
Press Triangle, Square, Circle, Square, X, X, Square.