During a few days or weeks, VMware should release their lightweight hypervisor VMware ESXi for free. It is an enterprise-class hypervisor with footprint about 32MB which is integrated into modern servers through e.g. solid state disks. The small footprint is achieved by dropping so-called Console Operating System (based on RHEL 3). It includes basic functionalities like vSMP or VMFS and for advanced ones, you need to manage it with VMware VirtualCenter. You can download it from here.