I've got:
* A Zero W hooked up to a PM2.5 to do air quality monitoring in the house. Just bought a couple more sensors for it (VOC, eCO2, etc), but haven't hooked them up yet.
* A 3B+ running the UniFi controller for my home network.
* One is running a custom Hue automation I built to shift the color temperature of the lights throughout the day.
* One is built into an internet connected dog treat dispenser I built as a gift.
* A rather dusty Pi is running CNCjs so I can have a decent interface to my cheap grbl CNC.
* And finally I have a Pi running OctoPrint for my 3D printer.
And that's just the ones currently running. I've got two more in progress. One to automate an exhaust fan based on inside and outside temperatures. Another is destined for the garage where it will replace the not-so-great MyQ "smart" functionality of the garage door opener.
To each their own I suppose, but I've been consuming RasPis like candy. $60 all-in gets you a fairly beefy platform with almost all the I/O you could require and a vast ecosystem of software and HATs. Honestly their only downside is that at some point I'll have to reconfigure my home network when I start exhausting my current internal /24 with 200 RasPis.