Donald Knuth. TeX. Also, pioneer of fundamental algorithms.

Elaborating on the TeX thing, it is said that all he wanted was a way to typeset his other works, and he was unhappy with all existing solutions because they produced bad-quality results, plus no one had yet thought of separating creating contents from formatting and layouting.

So he just went and did it himself. That's a spirit I admire!

And he ended up creating something that continues to be a standard these days. I know for a fact that large real-world publishers (I used to work for one) do use LaTeX to typeset the books they produce.