There are a number Content-as-a-Service offerings that can be used with static site generators. (Though not usually with GitHub's built-in Jekyll support as it doesn't allow for plugins)
Yeah, I love the idea of Contentful, I just cannot use it unfortunately. If Contentful was available as a purchase option for self-hosting, then I'd be all over it. But it is exactly what I need.
Examples: Contentful, Prismic.io, Built.io, CloudCMS
Full disclosure: I work for Contentful