I built an accelerometer-based washing machine sensor which sends a notification to my phone when the washing is done. This is useful because the machine itself has no indication when it's done: http://www.instructables.com/id/Washing-Machine-Notification...
I did a similar thing, via an esp8266 device and a vibration sensor! I also automated the display of tram-departures from the tram-stop next to my house with similar hardware. Otherwise I setup some "wireless buttons" to trigger alerts/actions on my Linux desktop system, and record local temperature/humidity into a time-series database for tracking purposes.
I was wondering about that as well but I only found the Amazon ones, which either need tricky operations to be usable (the brand-oriented dash ones) or expensive (the Amazon IoT (eom dash button) for 25 eur).
Surprisingly I did not find one on AliExpress