VirtualBox is a virtualization software that allows running several OSes on a single host machine. It was first a free VMware Workstation-like tools but has grown quite a bit now. You can now run virtual machines headless, like you do with Xen or KVM.
Here’s a little tour on setting up an hypervisor using VirtualBox on FreeBSD 9.
BTW: Why FreeBSD? Because it features ZFS filesystem version 5 and ZFS pool version 28.