VirtualBox
Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), Solaris and OpenSolaris, and OpenBSD.
VirtualBox is being actively developed with frequent releases and has an ever growing list of features, supported guest operating systems and platforms it runs on. VirtualBox is a community effort backed by a dedicated company: everyone is encouraged to contribute while InnoTek ensures the product always meets professional quality criteria.
The VirtualBox OSE binaries and sources are available free of charge under the terms and conditions of the GNU General Public License, Version 2.
More Info - Screenshots - End User Docs: VirtualBox.org
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