Gary Kildall could have become a household name and, possibly, the richest man in the world.
In 1980, IBM approached Bill Gates at Microsoft to license a BASIC interpreter for their soon-to-be-released Personal Computer (PC). They mentioned they also needed an operating system, and Gates referred them to Digital Research, Gary Kildall's company.
For various reasons, things didn't work out, and IBM went back to Microsoft. You know the rest.
There are various accounts of what actually happened, and the Wikipedia article on Kildall offers what appears to be a neutral report.
I used DR-DOS. I used GEM. And I wish more people remembered Kildall's contributions.
(YouTube video: Remembering Gary Kildall)