I've never used Prolog in the 15 years since I learned it in college. It's an interesting take on programming, for sure, and I appreciated the mind-expanding exercise, but hasn't helped me in my career at all.

Totally agree on awk. I use it almost every day for quick little one-liners. Big time saver.

Also agree on state machines, because from there it is a short hop to understanding formal grammars and the foundation of compilers and languages, which has been immensely useful in my career.