Big Head Mode:
International Superstar Soccer '98
Wait for the words "Press Start" to flash on the title screen. Now press C-Up, C-Up, C-Down, C-Down, C-Left, C-Right, C-Left, C-Right, B, A. Now hold the Z button and press Start. You'll hear the announcer shout to confirm the code.
Complete the International Cup on Level 4 or higher. Once you have viewed the end sequence go to Create Player. At the "head select" screen press the Left Shoulder button. This will take you to page 11 and a selection of strange and unusual heads.
If you finish the league at any difficulty and win, you can play against the World Stars. When you win the World Series at level 4 and defeat the World Stars, you'll have access to 5 more hidden teams. The teams are: Euro Stars A, Euro Stars B, African Stars, Asian Stars, and All-American Stars.
When creating a player, at the point distribution press C-up for more abilities.
Thanks to Revolution reader Sufyan!