Before I knew anything about electronics or engineering, I built an automatic fish feeder. I used a single MOSFET transistor that was controlled by the parallel port of a Linux server. It was very simple really; I mounted a motor with an offset weight to a small food hopper and when the motor spun the hopper would shake food into the tank. I also built a web interface for it that would allow me to feed the fish from anywhere with Internet access.
For the rest of my life I will remember how awesome that feeling was when my software lit up an LED in my dorm room back in 2001.