Whether you prefer Linux, Windows, or Mac OS X, you can probably get almost everything you need done with your chosen OS. However, sometimes a task demands an OS that you are not currently using. That’s where virtualization programs like Sun Microsystem’s VirtualBox 3.0 come in.
What is it? VirtualBox is an open-source virtualization program which lets you run guest operating systems with your native desktop operating system. For instance, if you need Windows to run Quicken, but prefer Linux for all your other work, VirtualBox enables you to bring up Windows and Quicken without leaving your Linux desktop.
What you get is an adjustable window containing the guest operating system floating on the host system. So, for example, you could have a Windows XP guest instance entirely hiding its Linux host system.
There are currently two editions: a full package that is free for personal use (enterprises should contact Sun directly); and the Open Source Edition (OSE), which lacks a few features such as USB support and an easy installer, but comes with complete source code. Both are free.