VirtualBox is a free, open source virtualization program from Oracle for the x86/x86_64 architectures.
From the VirtualBox Website:
VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product. 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).
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, Windows 7), DOS/Windows 3.x, Linux (2.4 and 2.6), Solaris and OpenSolaris, and OpenBSD.