Virtualization allows you to run operating systems on your computer that don't normally work on your computer. For example, using Parallels on your Mac, you can have a window that is running MS Windows. Using VMWare, you can run just about any guest operating system you want on just about any machine. While VMWare and Parallels are good products, the down-side to them is that they are relatively expensive for the home or occasional user.

VirtuaBox is a new virtualization system that aims to take care of that problem in that is is Open Source and totally free. It runs on machines with Windows, Linux, Macintosh and OpenSolaris operating systems and it supports a good number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.

While it's a little rougher around the edges than the commercially available solutions, it does work. And it's free. And they are actively developing, so look for it to get better and easier to install over time.