Server 2008 R2 has the Hyper-V technology, which allows multiple computers via their operating systems to be run simultaneously on one computer. They are, of course, hosted through Server 2008. The use of virtual machines has been one of the quiet innovations that may be ready to explode into the public mainstream.
Virtual machine software has been around for several years. It is a computer inside a computer. With VM software you can run Windows XP, Server 2003, Vista, Windows 7, all side by side in a sandbox frame and not interfere with one another. You can create networks inside a hosting computer. In fact you can even do it now with Windows 7. But that leads us to Windows 8.