TeX
The program that typeset modern science, down to the last equation.
Donald Knuth wrote TeX, the program that typeset modern science.
Donald Knuth
License on this page
License notice
The bars are main_control from TeX. A lawyer cleared this listing. The program and the name TeX remain Donald Knuth's.
Donald Knuth wrote TeX because a new edition of The Art of Computer Programming deserved to look as good as the metal-type edition it replaced. The program went on to typeset most of modern science.
The bars are main_control, the procedure that decides what TeX does next. It reads a token and switches on the current mode. Horizontal mode builds a paragraph, vertical mode stacks lines, and math mode is a grammar of its own.