Solar panel + Arduino
A couple of years ago I built a sun seeking solar panel. I made a very simply system design, I used a position servo to move the panels around and the output voltage from the solar panels as sensors. The brain for the system was an Arduino Uno which I programmed. From time to time I come back to this project and add some small features. Last year I programmed a control interface for it in visual studio (C#) and also enabled that it sends the voltage readings via USB to the computer.
It has been one of these things that I spent quite a few hours on without having any plans to make something valuable out of it. But I guess in the end it moves, it senses and produces data. What more can an engineer ask for? 🙂