For Science Olympiad, I built a running car powered by elasticity. I used rubberbands to wind around the axel and propel the car, counting each wind to determine the exact distance it would travel. It won 1st at regionals.
My friends and I built an app that detects gunshots and calls on immediate responses. It won 2nd in the Congressional App Challenge. You can view it here:
Ammo-Alert.com