What VM does FreeBSD run well under

[stan]

I am planing on rebuilding my laptop shortly. I am going to put Ubuntu 9.10
on as the base OS, and I want to be able to run various versions of FreebSD
as guest OS'es under one of the free (EG not VMWare) virtual machine
choices.

Which of these does FreebSD run well under?

-- 
One of the main causes of the fall of the roman empire was that, lacking
zero, they had no way to indicate successful termination of their C
programs.