VirtualBox is one of the best virtualization softwares available for operating systems. Basically, it allows you to run multiple operating systems in the "mother system", in various configurations and with various settings. This allows you to test the behavior of programs in different environments - check software compatibility with different systems, the detection of conflicts between different applications or for the study of their interaction with the operating system. Virtual machine can be installed "at will", thus it doesn't slow the performance of the primary operating system.
The program allows you to add to the system virtual USB devices and optical drives which are present in the core system. It is possible to perform snapshots of the current state of the virtual machine and use the machines in full screen mode (useful for all people who are making tutorials). Installation is simple and it literally takes just a few clicks to add a new virtual machine and make it ready to run
VirtualBox Open Source Edition was released as free software on the GNU General Public License which means you can distribute it and also use it for commercial purposes, meaning you don't have to worry about selling apps you make later. Currently, VirtualBox runs on both 32 and 64-bit Windows and Linux. It supports many types of virtual systems, including 4.0/2000/XP/2003/Vista/7 Windows NT, DOS / Windows 3.x, Linux (2.4 and 2.6), OpenBSD and Solaris.
Download page: http://www.virtualbox.org/wiki/Downloads