VirtualBox

VirtualBox is a family of powerful x86 virtualization products for enterprise and home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL)....
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VirtualBox has an extremely modular design with well-defined internal programming interfaces and a client/server design. This makes it easy to control it from several different interfaces at the same time. For example, you can start a virtual machine in a typical graphical user interface and then control that machine from the command line, or even remotely.