An automatic fish feeder. I was going on a long trip with my family and needed to feed my fish while we were gone. I remembered I had a servo from an old project. I drilled some holes in a round plastic container, glued it to the servo shaft, hung it off the tank with some wire, and wrote a small Arduino sketch to jiggle the container every 24 hours. It worked better than I thought it would; the fish survived, and in fact seemed so happy that I don't feed them by hand anymore.
I've since built a couple more for my other tanks, and I rewrote the firmware for ESP8266/ESPHome. Now my family can ask Alexa to feed the fish, because of course the world needs that.