Andrew S. Tanenbaum publishes Minix 3.3.0. Tanenbaum is one of the lighthouse open source folks out there (don’t forget: his last university lecture is coming up in the Aula of Vrije Universiteit – and we are invited for free coffee!).
Minix is cool, based on a microkernel of less than 13,00 LOC.
It is largely compatible with NetBSD, and runs thousands of packages. Does ARM, too. Go get it.