The alien operating systems that work in Raspberry Pi3
Riscos
https://en.wikipedia.org/wiki/RISC_OS
https://www.riscosopen.org/content/downloads/raspberry-pi
Plan9
https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs
9pi.img.gz
http://elinux.org/Plan_9_on_Raspberry_Pi
http://proness.kix.in/plan9/
Latest image
http://proness.kix.in/plan9/mirror/9pi.img.gz