For myself, I've built a web-based task manager. Nothing fancy, apache, PHP and JavaScript. It's tied into a number of the systems I have running at home, such as a similarly built budgeting platform that handles all our money and sends me a biweekly report of all the payments I need to schedule, and any checks I need to write (usually just rent.)
I've also setup some things for the wife, who's an artist on Patreon, where she uploads works she's finished to a given folder and my machine grabs them, resizes them to whatever she needs for different purposes, and emails them to her. It also applies watermarks depending on the appearance of it (light vs dark) and will also add it to her Jekyll website, commit the new image to the git repo, build and push it live with no interaction unless something blows up along the way (122 days since last workplace accident)
I also have a bunch of LED ropes, night lights, and our bedroom TV setup on smart plugs that are turned on and off automatically, but I don't know if that counts since I didn't build it, I just bought them.