Another one I made for my last job, and it's for annotating GeoJSON geometries: "Geometry Review".
You load a set of geometries to compare into it, and it shows a bunch of buttons for each set: "OK", "Bad" etc, your choice.
No extra clicking or studying required, you glance at the map, press a 1/2/3 key, see the next item. And export the result in the end.
Used this tool to annotate hundreds of routes and review predictions.
https://github.com/Zverik/geometry_review/
Geo is easy until you see it on the map.
For a very long time the leading repository on my github was "leaflet-grayscale" with a hundred stars.
I love @leaflet and made too many plugins for it. This one was quite simple — but I guess useful enough.
It just makes the map grayscale.
There are too few layers fit for a visualization background. Sometimes removing colors (and labels if possible) is good enough.
https://github.com/Zverik/leaflet-grayscale