GitHack is a free, OGL, OSR, setting agnostic sword-and-sorcery game. It was developed after StormHack, an OSR-like system that could be used to run Stormbringer-like adventures in Moorcock’s Young Kingdoms.
Even though you can freely collaborate on some indie games the static pdf format isn't optimal. This is a different approach to make it easier to collaborate by presenting it like a software project. The text is in markdown making it highly portable whilst supporting basic formatting and tables in a human readable way. You can then take the markdown text and convert it with pandoc or view it with your chosen markdown viewer.
Since this is a GitHub project you can clone or fork the repo and submit your own commits for the main branch. You can use Git from the command line and make edits with any text editor, see where your content differs from the main branch, and so on. However there are also graphical tools and plugins available. I recommend atom.io which integrates with GitHub nicely and has a lot of useful packages for writing and rendering markdown, and it's cross-platform.
Ralph Lovegrove (@fictoplasm)