Physics Calculator
The first unit we studied in my physics class in my senior year of high school was one-dimensional kinematics. At the same time, I had a computer science project in which we had to create a calculator app for our math or science class. I built out this calculator which solves one-dimensional equations.
Realizing how helpful this calculator is, I decided to publish it to the app store. It is available here for free. This is also a completely open-source project. Check out the code over on my Github.