Hit a Homerun: When you are in the batter's box and using the power swing press Up/Left + L2 + R1 + X when the ball is thrown and if the batter connects he will hit a homerun.
Unlock Super Player: Enter 'Scott Murray' as a player name at the 'Player Creation' screen. You can also enter any other development team name (which are located at the back of the instruction manual) as a player name.
Unlock Baseball Bloopers FMV Sequence: Enter the 'Start' menu and allow the MLB 2000 demo to run. Keep watching it until the MLB 2000 logo appears again and a blooper FMV sequence will begin.