Here are codes for NFL Gameday:
SKELETON
STEROIDS
CRUNCH.TIME
PICK.CITY
CANNON.ARM
BIG.BOYS
URNOTREDE
OFFENSE
DEFENSE
JUICE
STICKUM
GOOD
FAIL
SNAKE

Remember to enter passwords in all capital letters. Some codes are very 
noticeable while others only make subtle changes in game play. The only 
play you need in the game is the following. Go into the 'Run & Shoot' and go 
to the 'Quick Outs' play.Now, instead of throwing to the intended receiver, 
throw to the receiver that is assigned the triangle button. Note, you need 
to stay in the pocket for a while and let him run his route. Once he starts 
making the break for the corner you're in! 20+ yards everytime, 99% efficiency.

As soon as the NFL Gameday logo appears 
press: R1, R1, L2, L1, TRI, CIR, SQ, L1, L1, L2, R1, R2, L1, TRI, TRI. This will
give you an option screen where you can select several "Cool" things.

Extra Team:
At the opening screen, press the following keys in 
order: Circle, Circle, R1, R2, L1, L2, Circle, Square, Triangle,-Then
hold down L1 and R1 simutaniously for about 5 seconds until the 
new teams appear!