The source was apparently release quite a while although I just noticed it now: https://github.com/farbrausch/fr_public
License: 2-clause BSD
Language: CPP/Small amounts of ASM I think
No idea what OS it compiles on, maybe just Windows.
Was pretty impressed by it myself back when it was released.
If you haven't Heard of it Before, check out Wikipedia's article here: http://en.wikipedia.org/wiki/.kkrieger