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Playstation - MLB '99 cheats

Super player
Go to the player creation screen or spring training and enter any 
names from the programmer team as a player name (example: "Scott Murray").
You will find those names in the back of the instruction manual.

More homeruns
Create a player that is 60 inches tall, small build, and has zero
attributes.

Baseball bloopers movie:
Watch the MLB 99 demo at the start menu. When one half of the inning 
is over, then a blooper movie will begin. 

Single or double hits
While in the batter's box and using the power swing, push Down to
hit a single or double about 90% of the time.

Hit a homerun
While in the batter's box and using the power swing, push the following 
buttons when the ball is thrown: Up/Left + L2 + R1 + X
If the batter connects, he wil hit a homerun.