Today Microsoft has relased the Hyper-V Server 2008.
http://www.microsoft.com/servers/hyper-v-server/default.mspx
This is hypervisor only version, without Windows.
Donwload link: http://www.microsoft.com/downloads/details.aspx?FamilyId=5CEACA28-7AC0-4613-9227-D0FA337331FD