For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
Bash. It has enabled me to automatize a lot of sh*t I wouldn’t do just because it would take me a lot of time/efford to do. There’s a LOT you can do with a few simple scripts, a few examples:
…you name it…
…in most cases just by typing one word in the terminal. It was kind of a mindblow coming from the crappy window$ crappysystem eons ago.
I find that the least surprising, but ok…