Includes resources and facilities for documentation of different tools and various concepts.
A free and open source text processor for AsciiDoc. A solid alternative for Markdown.
A site wherein official documentation of different languages can be found; there's also an offline version of it if you are lacking of internet speed (like me).
Offline, easy to use, and usable on mobile devices. I dig it.
A free and open source collaborative wiki software without the use of a database.
A free and open source documentation tool to generate online/offline references for your written software. Mainly supports C++ but it has support for other programming languages too. If you want to see the code yourself, you can visit the repo.
A GitHub for documentations, enabling team-based documenation work much easier and more searchable.
A free and open source typesetting system for a wide variety of literatures from articles, books, and theses. It's the de facto tool for scientific and mathematical publications. You can get started on using it by downloading certain tools with it.
A free and open-source software (FOSS) that serves as a non-linear personal notebook. It can be hosted on GitHub and GitLab (and other similar web hosting services). Think of it as a personal Wikipedia.
A free and open source document converter. Can convert various document formats (i.e. LaTeX, JSON, their own variant of Markdown (and other Markdown flavors)) into various output formats such as HTML, eBook formats, PDF, and other TeX formats. You could also extend into writing your own custom format, provided you'll write one. If you're interested in finding the source code, you can see it from their GitHub repo.
I haven't used much of this tool but this is one of the tools that I look forward to adding to my toolbelt.