Freedom
I'm very much of the opinion that the basics of running a virtual machine should absolutely be free. The notion of paying for hypervisor is simply ludicrous. There's nothing to it. Management is another matter, as some of the pioneers of this latest wave of virtualization seem to have recognised some years ago, if you can integrate the management of these virtual platforms and environments into a broader suite of solutions it's real value add - hence it becomes a chargable effort, and something that I'm prepared to pay for.