Most useful is relative. The book dealers who gave me my first ever contract for an automated leaflet generator that generated customized leaflets for each dealer would consider that the most useful. It's been over a decade and they still use it every day. Some other projects that come to my mind:

- an open source OIDC authorization and identity server (Ory Hydra and Ory Kratos)

- a system that uses computer vision to track food waste in industrial kitchens, helping them understand and minimize food waste

- a live milk quality estimator for a milking robot

- a browser extension called memorize. I wrote this reusing code from another contract I did while still in secondary school, and I'm mentioning it thanks to a user named lush berry who wrote "literally the most helpful thing i ever found on the internet. it helps me memorize stuff even when i'm procrastinating which is amazing. however, i have a lot of suggestions, does anyone know how i can contact the makers of this extension?". I wish I had the time to listen to these users and make the changes they want.