A recently had to build a new server, really big server... 48 cores, 128gb RAM and 16tb of local storage. I wanted to use VMware ESXi server - the free bare metal hypervisor, but it is limited to 32 cpu's. I looked at Microsoft's HyperV and I have never been less impressed by a product. So I looked at Citrix Xenserver 5.6 - free version. It supports up to 64 cpu's and is a true bare metal hypervisor. There are desktop clients to manage from a workstation, it manages disk stores very niceley and has tools for all the guest OSes. It's not quite as friendly as VMware, but IMHO much more usable and easier to setup clusters.