Madden 98 Extra Teams
At the team selection screen hold L1 and L2 while tapping X, X

Extra Teams
Enter these names in the create player menu:
ORRS HEROES - EA Sports Team
LOIN CLOTH - Tiburon Team
LEADERS - All Time Leaders
COACH - All Time All Madden
PAC ATTACK - All 60's Team
STEELCURTAIN - All 70's Team
GOLD RUSH - All 80's Team
ALOHA - NFC
LUAU - AFC

Secret Stadiums
Enter these names in the create player menu:
JETSONS - Astrodome (Old Oilers)
DAWGPOUND - Cleveland Browns Stadium
SNAKE - Old Oakland Stadium
BIG SOMBRERO - Old Tampa Bay Stadium
DANDAMAN - Old Miami Dolphins Stadium
OLDDC - RFK Stadium (Old Redskins)
SHARKSFIN - Tiburon Sports Complex
GHOST TOWN - Wild West (late 1800's Texas)