https://www.sparkfun.com/products/13772
This is a muscle sensor, something that can tell if you are flexing a muscle or not. It seems like it could be very useful for things like disabled people who are missing a finger or something like that, as well as something for probably very advanced sports training or injury rehab.
https://www.sparkfun.com/products/13316
This is a voice recognition shield for an Arduino. I was not surprised thatvoice recognition existed for Arduino, but a dedicated hardware piece was surprising for me because I figured it would only be a microphone and heavy software implementation. This isn’t the case however, and it could be nicely used for making your own smart home like creations, like an Amazon Echo or Google Home, or even just shouting at your TV to be quiet. 
https://www.sparkfun.com/products/14045
This is a particle sensor, and I was very fascinated to read it’s description to see how it worked because I was not expecting this to exist, especially not in this way. It shines different colored LEDs and has a very sensitive photoreceptor on it, allowing it to be able to detect what the LEDs were shining on just by what was reflected. This could be very interesting to use, Just some simple things that come to mind are a smoke detector, a rudimentary food checker, or maybe even an air quality meter.
Berry College Physical Computing 2017 