Static site generators — especially ones that operate on a relatively simple file-backed content store full of Markdown or YAML documents — exploded in popularity as the complexity of enterprise class CMSs rose.
Netlify developed an interesting open source pseudo-CMS that splits the difference. Configuration files define the structure and rules around markdown, JSON, or YAML files that make up a site’s content, and a Javascript application acts as a lightweight front-end for editing those files.
DevelopmentSeed built something similar for the Healthcare.gov project years ago, and released it as Prose.io. The Jekyll team has rolled out a Jekyll-specific version as a plugin to its static generator. NetlifyCMS seems to be the first attempt to build one that’s generator-independent.