Oracle VM VirtualBox is an industrial-strength open source virtualisation tool that makes it easy to create virtual machines (VMs), simulated computers that run on your PC but act as though they were separate systems. It's a powerful capability that has many different applications.
If you'd like a closer look at Windows 8 before you upgrade, for instance, then you could install Microsoft's latest in a VM, then access it in a window on your XP or Vista desktop.
Or maybe you've upgraded to Windows 8 and find a favourite old app doesn't work any more? Create a Windows XP VM and you might be able to run it again.
VirtualBox can also be a useful security tool: if you download and test apps in a VM, then any malware you might encounter will be isolated from your main system.
And it's the perfect choice if you want to try out another operating system with the minimum of hassle. Right now you can install Google Chrome OS, all the mainstream Linux variants (2.4 and 2.6), OpenBSD, OS/2, ReactOS, SkyOS, DOS, and just about every version of Windows there's ever been. (Of course you'll need to have the system discs to hand.)
Note that Portable-VirtualBox will enable you to take the VirtualBox installation file, extract the contents and turn this in to a standalone portable application. This is not a portable version of VirtualBox, just a creation tool.