About a year ago, shortly after if was taken over by Sun, Innotek announced it was looking for FreeBSD developers. Their labours seem to have borne fruit, as a FreeBSD version of Virtualbox has been announced.
VirtualBox is a general-purpose full virtualizer for x86 hardware. Targeted at server, desktop and embedded use, it is now the only professional-quality virtualization solution that is also Open Source Software.
FreeBSD is not (as yet) officially supported, but Sun developers have been working on aFreeBSD version in their spare time.
Though there are still a number of features yet to be implemented and a few known bugs and problems to be squashed, Sun is looking for anyone interested to test this version, give them feedback and/or give a hand improving VirtualBox for FreeBSD.
In the effort to make VirtualBox the most portable platform for virtualization we can announce that VirtualBox is finally running on FreeBSD. http://www.virtualbox.org/wiki/Screenshots contains two screenshots of VirtualBox running on FreeBSD with a Windows and a Linux guest.
Even though the basic functions are working there are still a lot of things to do and we do not recommend to use it on production machines. Furthermore Sun does not officially support FreeBSD yet. This is a port done by developers in their free time. However if you are a developer or want to test it you can checkout the latest code and build VirtualBox yourself.