Bark detector
A tiny microcontroller that listens for loud sounds and calls a URL, so I get a notification when the dog barks and nobody is home.
An M5 Atom Echo (an ESP32 with a microphone, about the size of a sugar cube) runs a small sketch that measures sound levels and, past a threshold, calls a webhook.
On its own that would be noisy. The webhook goes to an Astromech script that also checks a Bluetooth presence sensor, and only notifies me when nobody is upstairs.