it runs raspbian: https://www.raspbian.org/
the pi port of debian and i have made some changes to it by default the non-free repo's are enabled in /etc/apt/sources.list and a few non-free programs are installed you can check if non-free debian packages are installed using vrms also and remove all the stuff it detectsbut still the program that start up the raspberry pi is non-free and apparently stops you from using decoding hardware for decoding mpeg-2 and VC-1 without a specific key
