Tracking Finances
I originally wrote a web app to track my finances in 2003 using classic ASP and T-SQL. In early 2017, I rewrote it from scratch, still using T-SQL, but with C#.NET and jQuery. Lets me review my budget, ensure my net worth is heading in the direction I want, make sure all my payments get made, and ensure my account balances never get too low (with a light forecasting element.)
Ideally I'd open source it, focus on the API documentation so anyone could write a back end, and iron out a few more front-end bugs, but since it gets the job done for me, the motivation never quite strikes me.