The original "ed" text editor, from 1969 Unix. Everyone should spend a few days trying to get some work done with it, if only to appreciate how we have nicer things now.
Another nice thing about ed is that it is sometimes easier to use than sed when you want to edit a file programmatically, since you can navigate lines at random (forward and backward directions), and you can still run regex find/replace like with sed. Just
printf 'i\nstring of ed commands\n.\n' | ed file-to-edit.txt
and pipe the commands into ed, although it is really an esoteric way to write scripts.