Share your favorite automations

I've been running HA for a while, and it's been working well; I haven't had to change much in a few months. That being said, it's fun to tinker with it, and I'm curious to hear what kind of automations the rest of the community is using. What automations are you most proud of? What are your favorite? What kind of interesting automations have you written?

My personal favorite is an automation that displays the current "apparent" temperature on a Hue bulb. It takes an average of the temperature, humidity, and luminance around my property and uses the average to compute an "apparent" (feels like) temperature. Then it applies a cosine function to the apparent temperature (to approximate how people feel temperature change), uses the resulting value to calculate a level between blue and red in CIELAB (a perceptually uniform color space), converts the results to RGB, and sets the color value of the hue bulb. The result is a bulb that changes color so that the change in color (as perceived by the eye) mirrors how the temperature "feels" outside. Ultimately what that means is that we can look at a small lamp with the hue bulb and say "It feels cold outside; we should put on a coat." It's probably overkill, but it was a fun programming exercise. We've started saying things like "It's really blue today, I don't feel like going out."

I'd really enjoy reading what kind of interesting automations everyone else has written.