Doxygen - Generate documentation from source code
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
DocFX - A documentation generation tool for API reference and Markdown files!
Compodoc - The missing documentation tool for your Angular application
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
swagger.io - Swagger is an open source RESTapi Documentation Tool.