Virtual Machines (VMs) category.  Some examples of VMs (in no particular order) are Oracle VM Virtualbox, OpenVZ, Microsoft Virtual PC, Microsoft Virtual Server, VMware Workstation, VMware Fusion, VMware vSphere, Microsoft Azure HyperV, Google Compute Engine, Red Hat Virtualization, Viruozzo, Citrix VM products, Huawei VM products, Sangfor VM products, etc.

Why is this category not a sub-category of Software? 
Because VMs, for the most part, require both a hardware component and software components, and so articles within this category could deal with either a hardware aspect of virtualization and/or software aspects of virtualization.

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