As Scott mentions, one VMWare license is good for as many virtual boxen as you have disk and memory for. The runtime footprint is ~96 MB minimum, and there's a system overhead you're not supposed to exceed, which generally restricts you to 3-4 concurrent virtual machines. Disk storage is dependent on the virtual machine, and can be as little as a few MB, though most "modern" OSs will want 2-4 GB.

VMWare multiplexes a single machine out many times -- you could easily have one or several score virtual configurations, with two or three live at a time, and the ability to "hibernate" a given image on request making swapping between virtual systems very straightforward.

Further, by configuring some "base" systems, with automated tools to install additionally needed applications or configurations, you can support huge numbers of configurations (readily thousands) accessible in a few moments, from a score or so base images. Storing each individual configuration is simply not practicable.

Another little-utilized feature of VMWare is to use it as a network testbed. Because the virtual machines can exist either on an entirely internal (to the host machine) network, or on externally visible network addresses, it's possible to set up and test complext network configurations, including firewall scenarios with "inside", "outside", and gateway system. All this without needing to seperately configure and set up physical boxes. I'll grant that if you're using VMWare strictly to provide access to another OS's applications, you're not getting the full value of the tool. But that doesn't make it worthless by any measure.

My only gripe on VMWare is that virtualization, particularly of IO, can result in slow performance in some situations. The delivery of fully independent, virtualized systems, which act fully autonomously, is above expectations.