Three that have been really beneficial, and all support/build on a hobby / volunteer effort of mapping mountain bike trails:

This one generates maps from OpenStreetMap data + some custom curated info in YAML: https://github.com/c0nsumer/trailmaps.app-map-generator

This one converts a basic chunk of OpenStreetMap data to an SVG so I can mark it up (by hand) in Adobe Illustrator to make specifically-styled print/PDF maps, such as what get installed at trailheads: https://github.com/c0nsumer/osm_to_ai

This one takes GPS recorded rides and builds custom/personal heatmaps serving up the map tiles so I can use them in map editing software: https://github.com/c0nsumer/local-heatmap-tile-server

And all of this has been put together to make the custom, local, specific-use-case maps that are at https://trailmaps.app (which, via local curation, are overall better mobile/online maps than many of the bigger auto-generated systems such as Trailforks, Gaia, RideWithGPS, etc, for visualizing local systems).

It's neat stuff where I understand all the inputs, outputs, and how most of it works, but AI tooling (Claude, mostly) has allowed me to bolt it together much faster than I would have writing it myself.