I don't know if it qualifies as the "most interesting" but as a travel bug I wrote a little airfare scraper that I got plenty of value out of.
Basically it's a script that scrapes several places for flight deals and "mistake fares" and notifies my phone if it matches with my city. No searching like other flight apps, you basically just set it up and wait, and various places will pop up. Helps if you're in a hub city e.g. NYC.
The phone notification was crucial because such deals sold really fast. In case the booking didn't work out I was usually covered by the 24 hour cancellation rule [0][1].
I started off running it on my own server but later I learned IFTTT handles device notifications without paying the Apple Developer tax, so I migrated things there. Used it more when I was single but nevertheless it's helped me land some killer deals e.g. NYC-Dublin RT for $300, NYC-Paris RT for ~$400.
Thought about making a paid app out of it but the limited seating and time-sensitive nature of these deals is tricky.
[1] I believe this rule, which is not so broadly known, was imposed on air carriers in response to certain fraudulent online marketing practices going on at the time.