VMware
 

How good is VMware????
How useful would it be to run linux as a guest operating system inside the Virtual machine????
is it an easy task for the user??

I've used VMWare since their early beginnings and it's a decent program. It's no harder to run Linux (or Windows/etc) than if you were to install and run the OS on its own PC.

It has been most useful for testing. Say you don't have extra boxes lying around and want to test some new kernel/dameon/utility without potential to damage your current box. You can emulate almost your entire system (sans specific hardware - VMWare emulates things like disks, sound cards and video) on an undoable or throwaway instance of VMWare.

The "undoable" disk feature is great for testing. Nothing like the ability to completely erase a complex set of changes and start fresh by "rebooting" the OS ;)