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VirtualBox is a powerful x86 and AMD64/Intel64 virtualization hypervisor for enterprise as well as home use. A hypervisor allows your computer to host one or more Virtual Machines, running different operating systems than your own computer runs. This is essential for all our projects (except for Sixtrack).
Not only is VirtualBox an extremely feature rich, high performance product, 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) version 2. Please note that the VirtualBox extension pack is not required with LHC@home, and not recommended, as it is under a different licence.
Detailed question
What is VirtualBox?