O'Reilly Atlas
O'Reilly Atlas is a web application aimed at simplifying the creation and distribution of digital content. It is a Git-backed writing and production tool that exports books as PDF, ePub, Mobi and HTML, from single source Markdown, Asciidoc or HTML files, styled with CSS only.
All features in the application can either be used through the UI, or through an extensive API that allows publishers to use Atlas as a part of their native toolchain.
A closer look at the app architecture and workflow can be found in this excellent blog post by Sanders Kleinfeld, or via the splash page of the application itself.