The ongoing licensing controversy surrounding the ‘libogc’ code, used by many Nintendo game emulators, has led to a discussion on Hackaday of reliable open-source code that might be useful to hackers but is hidden away.
One example is RTEMS, a BSD-licensed operating system developed for aerospace, which is also popular among particle accelerator systems.
Another is NASA’s Core Flight System, which underpins much of the agency’s spacecraft, although since NASA opens much of its code to the public this is more in the realm of ‘hidden in plain sight’.
The Hacks section on Hackaday is worth exploring for further similar examples, and the discussion continues here.