jekyll and hugo are the 2 big ones I know of. They turn markdown into html (and also do a bit more).
Hugo is architecturally less flexible, but you get sensible things, like autogenerated index pages, while themes are just template/html/css bundles.
Jekyll is more flexible, but also comes with a lot less. I spent way too long trying to figure out how to generate index pages (still don't know how) or get it to recognize that I want my directories relfected in site navigation structure. It's ruby, "themes" are more like fully fledged plugins and I don't know why you would want to use it.