If you'd like a closer look at Windows 7 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 7 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.)
New additions to VirtualBox 4 include an interface redesign, making it easier to view and manage your virtual machines. VM displays can now be scaled, so you can reduce a window size by half (for instance) and still see everything that's going on. The ability to limit a VM's CPU and IO time means the program will be less of a drain on your system's resources, and there are a host of other performance optimisations and bug fixes available