MINIX 3 is an operating system designed to be highly reliable, flexible, 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.

Homepage:
http://www.minix3.org/

One of the formats in which minix 3 can be downloaded is as a VMware-VM ready to run in the VMware player.