I just learned about a very useful keyboard shortcut, apparently introduced with macOS Sierra:
In the Finder you can toggle the display of hidden (invisible) files with ⇧⌘. (Shift-Command-Period).
With previous OS X versions this shortcut used to work only in Open/Save dialog windows. To show hidden files in the Finder you had to change the Finder’s settings via defaults write in the Terminal.
Strangely, as of March 2017, the shortcut is not documented on Apple’s Keyboard Shortcuts page.