It's sad how almost nobody teaches old UNIX goodies these days. I don't remember how I learned about man page sections but certainly nobody taught me. Some people on Reddit discovered script(1) after I used it to crash grsec. What's your favorite less well known UNIX tool/trivia?
In bash, M-. (<esc> .) or M-_ (<esc> _) when in vi mode (set -o vi) to insert the last argument to the previous command. Also...vi mode.