I've been a big fan of VMware's virtual products since they were first released. I've been a user and beta tester of their VMware Workstation product since before the original v1.0 release and I still use VMware today.

Not only is VMware good for testing different operating systems and applications in a safe environment, but it is also an excellent way of testing entire networks (for example, when testing grid applications) and, in a server environment, it becomes an excellent way to consolidate multiple servers onto a single piece of hardware and achieve better server utilization and eke more power out of fewer pieces of hardware.