I've never met Michael Spivak, so don't know much about him, personally, but I think many people would consider him a geek. He is a calculus professor whose textbook was widely adopted for many years; two classmates of mine in grad school, from widely separated parts of the USA, had used it. If that wasn't enough to qualify him, consider this: Many geeks like to take things apart, see how they work, and put them together in a different way. Spivak's research field is differential geometry, which uses calculus and algebra to study geometry. Not quite like taking apart a streetcar to make a rocket sled, but still in the spirit of geekitude. He also wrote a guide to using TeX, an enormously complex and hard-to-learn text formatter aimed at getting every black spot of a mathematical formula in exactly the right place.
He also displays the kind of quirky I-don't-care-what-others-think attitude some geeks have. In the front matter of that textbook appears the phrase "Dedicated to the memory of y.p." If you look in the index, you might trip across the entry "Pig, yellow." There are only three references: to the dedication page, to the index page (self-reference being a favourite thing of many math geeks), and to a page in the middle of the text that does not contain the phrase "yellow pig". You have to apply abstraction, another math geek thing, to realize that "whole hog" counts. That guide I mentioned above was called "The Joy of TeX," a similarly quirky name to call a technical manual.
I'd like to think Spivak will be remembered for a long time -- and since he was born in the 1940's he likely has some good years left. His whole body of academic work counts as a good legacy among geeks, but textbooks go out of fashion, and the undergraduates you mentored go on to other things. I wonder if Spivak might actually be remembered longer for something that has nothing to do with mathematics.
Some of us don't like using "he" to refer to both men and women, like every English speaker used to do a hundred years ago. When I wrote a textbook back in the 1980's, the guidelines from Prentice-Hall gave several suggestions, like alternating "he" and "she" in different examples, or using "they" (which actually has a fairly long history as a gender-neutral pronoun). I've sometimes used he/she. I'm not happy with any of those, for reasons that aren't particularly relevant at the moment. So, when I started contemplating writing again, I asked one of my kids (who was taking Gender Studies at the time, and who had no idea Spivak was also a math prof) and ey said the Spivak pronouns were the best choice.
"Ey:" not a typo, Firefox. Basically, the current incarnation takes the 3rd person plural forms and eliminates the "th:" ey, em, eir, emself instead of they, them, their, themselves. This has the distinct advantages of (a) clearly distinguishing singular from plural and (b) being trivial to remember once you learn the drop-the-th rule. Spivak says he heard them from someone else, whom he doesn't remember, but he certainly popularised them (which might speak to the cred he has as a mathematician, among university audiences). I suspect that by now if anyone figures out who originated them, they'll still be called the Spivak pronouns.
What strikes me is that it's a geekish way to revise the language: take a look at a structure (albeit a simple one, and linguistic rather than technological), take it apart, and use it in a different way. Their simplicity might mean that one day they become a core part of the language everybody uses (at least, the ones who don't reject gender-neutrality outright).
Not a bad legacy for a geek.