I came in looking for awk in the list to upvote. By far awk has saved me the most time. I have used it for everything from parsing EDI documents to making simple shells for managing processes.

I just used it to create a bit over 2000 data transformations and wrote another awk script to tie them all together. Yeah I did in hours what would have taken a week in the GUI and changes will be able to be made in minutes instead of days. Plus it saved my wrist from the pain of clicking a mouse thousands of times.

To be fair you can do the same thing with any programming language but awk just has the least friction for me.