With all of my family being at home the last few weeks and me working remotely, I decided to try something fancier than a sign on the doorknob. The sign has been working just fine, I just needed an excuse for a Nerves project.
This project uses a strip of Neopixel lights to indicate if I'm in a meeting or free. It runs on a rpi0, but would work with any similar hardware. There is a web page with two buttons which will change the indication - Red if I'm busy, or green if I'm free.