The New Tetris Codes -

Turbo mode:
Select single player mode, enter 2FAST4U as a name, then highlight "OK" and press A. When the game begins, the blocks will eventually fall very quickly.

Turbo CPU mode:
Select single player mode, enter AI2EZ4U? as a name, then highlight "OK" and press A. When the game begins, the CPU's blocks will fall very quickly, while your blocks will remain at normal speed.

Music kaleidoscope:
Enter the audio options screen, set the song to "Haluci", and the music mode to "Choose". Then, select single player mode, enter HALUCI as a name, highlight "OK", and press A. A kaleidoscope will appear in place of the game. Note: The Nintendo 64 must be reset to quit this mode.

Delete line totals and reset Wonders:
Select single player mode, enter O1DERS as a name, then highlight "OK" and press A.

Delete line totals, reset Wonders, erase high scores:
Select single player mode, enter 1N175R4M as a name, then highlight "OK" and press A.