Ask HN: Favorite Free Web Tools?

I have a collection of favorite smallish web tools that I use. I was wondering what tools other people use? Below are the ones I use the most:

For diagramming | https://www.yworks.com/yed-live/ For PDF editing | https://simplepdf.eu/ For differencing | https://www.diffchecker.com/ For geography | https://geojson.io/