Donald Ewald Knuth, best known for his masterwork The Art of Computer Programming (TAOCP) was greatly displeased with the computer-composed galleys for the second edition of volume 2 of TAOCP and believed that, as a computer scientist, he could do a better job. His response, TeX, Metafont and the Computer Modern fonts, took about ten years away from his work on TAOCP, but provided a stable platform on which he could reliably continue his writing, free of concern for stability and changes in hardware and software. He released it as free software, one of the first major contributions to that field.
This legacy has benefited hundreds of other technical authors (a group not well served by commercial suppliers, for whom it is too small and difficult to guarantee a profit). His custom is to reward every "first finder" of a software bug or erratum in a book; being recognized as such a finder is considered a great honor, and many reward checks have gone uncashed, being instead held for bragging rights.
For his own overview of TeX and friends, here is his TeX/MF web site. You might also enjoy the panel discussion from TUG 2010 with Knuth and almost all the original Stanford TeX Project members, and Don's "Earthshaking Announcement" of what the future might hold.