As voretaq7 mentions, VMWare Mircrosft Hyper-V packages are good choices for indutrial strength virtualization packages. But at the same time, I personally am using VirtualBox in our production enviromnet as a secondary Active Directory/DNS server and has been working pretty solid thus far. It's great for those that are teetering on Virtualization, but not quite sold on the idea. Those industrial strength, however, are a little more involve in setting up (not by much) and are great once you have decided to bring virtualization into your enviroment. VirtualBox is nice because its supported across almost all OS's and can bring up a VM on almost PC that is capable in terms of hardware.