Is there a free hypervisor that runs on a Mac Mini? I need to run 4 instances of Mac OS X desktop (10.4 to 10.7) for our continuous integration setup (so they need to be on all the time). I've used PC hypervisors in the past (XenServer, ESXi, etc) but never for a Mac.

Is it possible to run those guest operating systems on a Mac Mini hypervisor?

Edit: Ideally, it should be managable remotely (like XenServer and ESXi), desktop virtualization software (like VirtualBox) isn't really what I had in mind.
You might take a look at VirtualBox which includes a GNU GPL version as well. It offers limited/experimental capabilities to run virtualized OS X VMs, but I think Apple generally forbids virtualizing OS X via their licensing/usage terms.