> Minix 3: still around, great resource for learning, especially since there is a great book about it by Tanenbaum himself. Btw: it utilizes NetBSD's pkgsrc infrastructure, so quite a lot of applications should run

The current Minix source tree (after integrating NetBSD) doesn't resemble what's in the book much. Even Tanenbaum has commented that they perhaps went too far in pulling in so much of NetBSD.