The only free, multi-platform, GPL visualization software that I know of is Virtualbox.

VirtualBox platform packages include,
o VirtualBox 4.0.10 for Windows hosts x86/amd64
o VirtualBox 4.0.10 for OS X hosts x86/amd64
o VirtualBox 4.0.10 for Linux hosts
o VirtualBox 4.0.10 for Solaris hosts x86/amd64

Virtualbox online documentation
User manual: PDF or Online

End User Docs: User FAQs - HOWTOs & Tutorials - Guest OSes

Technical Docs: Including SDK Programming Guide (API reference), FAQ, Binaries overview, etc...

For an OS, the best would be Linux. I like the Ubuntu distro (version) of Linux. The regular Desktop Ubuntu that the most use is a little bloated. Fortunately there is a minimal install of Ubuntu you can get here. Out of the box, the min install might not be ready for BOINC but adding the needed files/libraries/updates is not that difficult. Also with Virtualbox, once you have a GuestOS set up just right, you can clone it.