Ask HN: Data workers, which topics or tools have provided you the most utility?

Data workers => People who deal with moderate to huge amounts of data for different purposes. They may be grad students, analysts working for some organisation, engineers, basic science people (biologists, mathematicians, chemists, etc), finance people.

Which tools or subjects that you have learnt proved to be most useful in your work? Where did you learn about them? Why do you find them useful?

N.B.: Books, courses, videos, docs, github codebases, anything you have shoot them at us.