Recent discussions on Twitter and in a few classes brought up the venerable VI editor (or the more common upgrade to VI, VIM). 

It was once a must-learn tool for Unix/Linux/BSD administrators. It was used for editing configuration files, data entry, and even coding. 

These days it's not quite the must-have it used to be. In fact, most people working in the industry can get by with the less capable, but far easier to learn, nano editor.