Sponsored by Endeavor Systems, Inc.
OpenFISMA documentation is version controlled just like the source code, and we manage it in trunks and branches like the source code, too. The documentation is written in MultiMarkdown format, which is a line-oriented, text-based format that is easy to read even in its raw format.
Any time a project member updates a piece of documentation, our continuous integration server converts the documentation to multiple formats (such as HTML and PDF) and publishes it on this website. This keeps the documentation fresh and makes it more useful for everybody: business analysts, evaluators, project managers, and developers.
If you're looking for API documentation, take a look over here.