I have two!

One is a webapp I wrote almost 20 years ago for my dad and it's still being used today. It runs on IIS and built with asp classic and vanilla html/css/js (no frameworks back then). They use it to track orders and invoices to suppliers/vendors and ensure what they receive is what they ordered.

The other, an electron-type app that saves people hundreds of hours per month by letting them bypass some bad UIs and interact with external services directly. It's been running for 6 years, only had to make very few updates, and it's the one thing I don't need monitoring for - not only it's been quite stable, I get called immediately if it breaks (eg when external services change their endpoints).