The most useful one I've done is opening the garage door from my phone or Google Home (yes, anyone can stand outside my windows and open the door on voice command, but noone would know to do that), and notifying me when I left it open for 5 minutes. I used a Sonoff relay switch for this, coupled with custom firmware based on homie. A LOT of hours went into this, but it was also a lot of fun and learning.

I've also made temperature and other sensors using esp8266 boards, but that wasn't quite worth the time spent.