Over one year ago I found an alternative keyboard layout/keymap for US keyboards, to help type accents, umlauts, and other diacritics, as well as Greek letters, mathematical symbols, subscripts and superscripts, etc.
It looked super handy, but I failed to bookmark it so now I have no idea what it was called. I remember it was written by a German (or French?) speaker. I routinely write in multiple languages + mathematics, so this could really be a productivity booster.