Kobe Bryant's NBA Courtside
Big Head Mode
Start a game in any mode. Press start to pause the game, then press the following buttons in this order:
Right on the Control Pad (not the analog stick), Right on the Control Pad, Left on the Control Pad, R, Z, Start, A, Start, A, Start, Z. Don't worry about what happens as you enter the code, just keep pressing the buttons until the entire sequence is complete.
Credit goes to: Steohen Anderson via IGN 64
 Big Head Mode
 Alien Invasion
 Small Players
 Extra Teams
 Disco Floor
Alien Invasion
First enter the Special Teams code below. Now go back to the main menu and press C-Up, C-Down, C-Left, C-Right, Start, Start, A, B, A, R and Z. Now select the Left Field team. When the game begins, the Left Field players will be changed into bug-eyed aliens.
Credit goes to: Eiksirf via Nintendo Power

Small Players
On the main menu press C-Right, C-Right, B, R, R, C-Down, Up, Down, C-Up, A, Z.
Credit goes to: JBond2686 via GameSages

Extra Teams
Play as any one of three hidden teams in the game by holding down the L Button on the main menu while selecting Pre-Season. Then just cycle through all of the teams listed and you should find three extra ones!
Credit goes to: Nintendo Power Source

Disco Floor Code
Start a game, then press pause (Start) and enter the following button combination:
A, C-Up, Control Pad Down, Control Pad Up, C-Down, R, R, B, C-Right, C-Right, Z
Credit goes to: IGN 64