Today a new product from Microsoft; Hyper-V Server 2008, became available for download free of charge.  This is the same Hyper-V technology available as a role in Windows Server 2008, but it's important to understand that Microsoft Hyper-V Server 2008 is a stand-alone product.  It operates with no GUI, which is similar to Windows Server 2008 running Server Core mode with the Hyper-V role.  Of course any guest operating systems you host in Hyper-V it will require the appropriate license.