Quite honestly the most valuable tool I'd learned in the 2010s (having been a significant Unix / Linux user since the 1980s). In the late 1990s / early aughts,

Quite honestly the most valuable tool I'd learned in the 2010s (having been a significant Unix / Linux user since the 1980s). In the late 1990s / early aughts, I'd written a toolchain to generate multiple document formats from a source based on HTML fragments. With Pandoc (and usually Markdown, occasionally LaTeX), I've discarded all of that, and have greater utility (more outputs, and FWIW, more input formats as well if I choose those).