Geojson.io is a simple tool that does its job well. As you draw shapes and place pins on a world map it produces GeoJSON code that you can edit and copy to save elsewhere. You can paste in GeoJSON to see its shapes drawn, as well as export to different file formats.
Mostly I use geojson.io to trace routes of hikes, bicycle rides, and car trips, both for measuring distance as well as for a personal archive.
This blog has prompted a couple uses. Once I wanted to trace a path on a reference image and get its geographic coordinates. I didn't find any metadata on the map about what projection it used, so on geojson.io I loosely traced Africa and Greenland (an equatorial landmass and a polar landmass) and projected the coordinates using different formulas to see how well my outlines aligned with Africa and Greenland on the reference image.
Another time I narrowed something down to a particular range of latitudes, drew a bounding box on the globe using geojson.io, and from that narrowed down the likely range of longitudes. I took a screenshot of resulting rectangle and included in the post, though I could also have saved the GeoJSON data to a GitHub Gist and embedded it, as I did when mapping Michigan county boundary roads whose names are portmanteaus.