Virtualization is where the action is !

Virtualization is the buzzword in the world of Operating Systems these days. Recently KVM - Kernel based Virtual Machine capability was introduced into Linux. When completed, it would make it possible to run Windows ( maybe other OS too) as a guest OS on top of Linux in the newer Intel and AMD processors that have support for virtualization.
KVM is virtualization specific to Linux. Other virtualization technologies also exist some of which like VMware are very advanced and allow many more OS's as hosts and guests.
Another virtualization technology under development is Xen which will be a real competitor of VMware as it will have support for many OS's just like VMware and can be used with older processors as well. Xen is an open source project unlike VMware which is proprietary.
Then there's hardware virtualization which allows one set of hardware to run many OS's. UltraSparc T1 aka Niagara is supposed to get Logical Domain ( LDom ) support in the near future which will allow one Niagara processor to run many different versions of Solaris OS simulaneously.
IBM and Sun have had hardware virtualization in their big iron for a long time but now even smaller machines can have it. Solaris for example allows a form of virtualization with Zones where a machine with Solaris 10 or some OpenSolaris distro can run dozens of virtualized instances of the the OS. Each Zone is a secure virtual OS instance on which applications can run which can be compromised without compromising other zones in the same system.
With all these different virtualization techniques in Unix, Linux, Mac OS X and even in Windows, user today is the king! What was unthinkable a few years back is now possible thanks to all the advances in technology, be it open or proprietary.