As recently announced Minix 3.0 has recently been released.  The original Minix was an educational operating system - that actually served as the inspiration for development of Linux.

From the Minix 3.0 website:

MINIX 3 is a new open-source operating system designed to be highly reliable and secure. It is based somewhat on previous versions of MINIX, but is fundamentally different in many key ways. 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.

One of the nice things about Minix 3.0 is that unlike previous versions of Minix - it installs and runs just fine inside of Virtual PC:

Minix 3.0 under Virtual PC   Minix 3.0 under Virtual PC   Minix 3.0 under Virtual PC

Cheers,
Ben