VirtualBox is a powerful x86 & AMD64/Intel64 virtualization product. It allows you to run multiple operating systems on your computer, at the same time.
VirtualBox can run 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, OpenSolaris, and OpenBSD. Virtualbox may also run modified versions of Mac OSX, though the operating system is not officially supported.
There are currently 2 versions available (see http://www.virtualbox.org/wiki/Editions):
(A) Full VirtualBox package in binary (executable) form, free only for personal use and evaluation.
(B) The VirtualBox Open Source Edition (OSE), released under the GPL and comes only with source code, not in executable form. It is functionally almost equivalent to the full VirtualBox package, but has no USB-support.
Virtualbox is available in 28 languages.
Portable-app version: http://www.techbeta.org/category/portable-apps
