VirtualBox is a free, open-source virtualization program. It allows the installation and use of guest operating systems from within your main OS, making it easy to use programs on a variety of software platforms without the need to set up a complicated dual-boot configuration. Advanced features of modern processors are supported, to provide the best performance possible even when managing multiple virtual machines - but some things like advanced 3D graphics are limited on the 'guest' operating systems.
VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product for enterprise as well as 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) version 2. See "About VirtualBox" for an introduction.