Operation Systems

Porteus:

http://www.porteus.org/

Android:

https://source.android.com/index.html

Thinstation:

http://tsomatic.agarwal-associates.com/

Uclinux (embedded):

http://www.uclinux.org/

Lakka (game emulation):

http://www.lakka.tv/

Kali (security tools):

https://www.kali.org/

Debwrt

http://dev.debwrt.net/

Chrome OS

http://getchrome.eu/download.php

Small IOT OS:

http://hackerboards.com/open-source-oses-for-the-internet-of-things/