Electric Book is a Jekyll-based tool for producing print PDF, digital PDF, EPUB, website, and app versions of books from a single markdown, YAML, and HTML-based content source. It was developed by consultancy and service provider Electric Book Works.
Manuscripts.io is a next generation writing tool to follow up Manuscripts.app (Manuscripts for Mac) re-engineered to support collaboration, whilst still allowing for a fully offline workflow, with both web and native desktop app client applications in the works (with beta version of the web client available).
nbviewer is a web application that lets you enter the URL of a Jupyter Notebook file, renders that notebook as a static HTML web page, and gives you a stable link to that page which you can share with others. nbviewer also supports browsing collections of notebooks (e.g., in a GitHub repository) and rendering notebooks in other formats (e.g., slides, scripts).