The Nintendo 64 Bio Sensor
You can actually buy this from Retrogamecity. Weird.
"This very odd accessory can be used on a Japanese N64 to measure your heart rate. Put the controller pak in, clip the sensor to your ear and your Nintendo 64 turns into a heart rate monitor!For use with games such as TETRIS 64. The game gets harder and faster as your heart rate goes up! Makes for some interesting gaming that`s for sure."