How did I miss this? Minix, the small OS designed by professor Andrew Tanenbaum to teach operating systems concets, is now in its 3.0 revision. From their webpage:

MINIX 1 and 2 were intended as teaching tools; MINIX 3 adds the new goal of being usable as a serious system on resource-limited and embedded computers and for applications requiring high reliability.

Neat. I’ll have to check it out.