I've been doing a lot of virtualization work over the last year or so. But, I have to admit that I never hard of VirtualBox before this week.

VirtualBox.org

It looks like it was a for-fee proprietary product prior to January 15, 2007. But, as of today, is a Free and Open Source Software product.

It hypervisor appears able to run on top of Microsoft Windows or Linux (but not Mac OS X). And, it can host various flavors of Microsoft Windows and Linux as Guest Operating Systems.

I guess Microsoft's Virtual PC (which is a freeware product now), Parallels for Windows and Linux, and VMware Workstation has some competition now.