I waited for a little bit to post this, but I recently ventured into the realm of writing PAM modules, and my first project has been finished. This is a MongoDB authentication module. In short, it allows you to authenticate users to a MongoDB database that the user has access to (for added security, the user should have read-only access to the database). This module requires libmongo-client, libpam, gcc and ld to compile (the last three should be on every system that allows for compiling programs). This module is written in strict C, and compiles without warnings. I’ve written a good how-to and such on GitHub, with the project officially supported by Zorveo (both links at the bottom of this post). Once you have libmongo-client downloaded and installed, set up is very quite easy. More modules will be released in the future as well.
Before asking for support, please read the README file on GitHub, as it was written to be as extensive as possible.