The name in software virtualisation, VMWare allows you to run an operating system in a virtual environment inside another operating system.
When running on a host, the VMWare software emulates a hardware environment, 'fooling' the guest operating system into thinking it's installed exclusively on a real hard disk, in the usual way.
Thus you can have a situation where FreeBSD runs in a virtual environment on an XP Workstation. Unix in a Window. Scary.
If resources permit, you can even have multiple OSs running at once.
Here is an image of FreeBSD Unix running under Windows 2000.
VMWare is just great for software and OS development work. I've used it to test a Linux FTP boot floppy installation, booting from within a virtual machine, DHCP and IP Addressing, help desk and teaching environments and interactivity between multiple operating systems, to name a few.
By pressing the F2 setup button during a virtual OS boot, you can even access a virtual BIOS too. So now we can have BIOS screen shots and the ability to screen capture literally any part of an OS installation sequence.
VMWare have recently made some of their packages free for evaluation use
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