ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
GitHub Desktop - GitHub Desktop is a seamless way to contribute to projects on GitHub and GitHub Enterprise.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
Git - Git is a free and open source version control system designed to handle everything from small to very large projects with speed and efficiency. It is easy to learn and lightweight with lighting fast performance that outclasses competitors.
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
Mercurial SCM - Mercurial is a free, distributed source control management tool.