Below follows a list of pre-built virtual machines, ready to run on VMware's Player, which can be downloaded for free.
Evaluate the Bricolage CMS running on Ubuntu. Bricolage 1.10.3 VMware image
A virtual machine with the MINIX 3 Open Source operating system. The MINIX 3 Operating System
TU Dresden Research Operating System. TUD:OS Demo Download page
Note: list under construction, see also the Virtual Appliance Marketplace. If you know of virtual machines not on those lists, please post a comment.