The following is a list of all open source RPGs I've come across:

Chronicles

Chronicles is a completely open source RPG, written in Markdown. The plain text base allows it to output to epub, pdf, and html.

Knave

While Knave has a licence compatible with the GPL, and other open source licences,

Knave, like many other RPGs, uses a Creative Commons 4.0 licence. But unlike many others, it actually has a source file which comes with the pdf.

Unfortunately, the Creative Commons only require redistribution of the output files. This means that when someone created the Rascals RPG, the resulting pdf fell under the same Creative Commons licence, but did not give anyone the legal right to also receive the source files. So if you want to modify the Rascals RPG, you will have to guess at every change made to the Knave source file, and then make those changes.

Cairn

Like Knave, Cairn comes with source files when you buy it. Those source files are, unfortunately, Affinity Publisher files, however, this still counts.

Luckily, Cairn's Github page seems to have all of the basic source files required for a game.

Siren

Written in LaTeX, Siren provides the classic open source experience. Unfortunately the writer abandoned the project four years ago.

BIND

Of course.

Definitions

'Open Source' refers to source files which remain open to change by anyone. They have no restrictions (except a few minor stipulations, which will not stop anyone from changing the source files, and printing off a new book).

Reasons for Disqualification

• Claiming to have an open source RPG, then sneakily handing over a plain text file, instead of the RPG and its source files.
• Disallowing someone from modifying the book, by including some files with a proprietary licence, e.g. Basic Fantasy or Open Adventure .
  • Open Adventure is particularly bad, as the repository comes with a LICENCE file, which states that the repository is under a Creative Commons licence. The book's intro, however, contradicts this, by mentioning that the art is not covered by this licence.
• Using the Open Gaming Licence, because the OGL is not open source .

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So that's it. I've found five open source RPGs in English, and I've searched a lot.