Windows/Mac/Linux: VirtualBox 4.0 makes creating virtual operating systems a lot less bothersome. The interface is easier to get around, the virtual machines easier to move or wipe away, display and hardware compatibility is improved, and new "extensions" can add new capabilities.
Previously, moving your virtual machines around, or deleting them and all their snapshots, required some knowledge of how VirtualBox stored your stuff, and some tricky XML editing. No longer, as VirtualBox adds commands to do a complete wipe of a machine, and also explains where your stuff is stashed. Beyond that deep-down fix, the interface has seen a pretty nice upgrade — stuff you would regularly look for before, like the prompt to load up an ISO file as a virtual CD drive, are now front and centre, and the Settings dialog easier to move through progressively.
There aren't many Extensions right now, but third-party coders may soon add some nice capabilities to this free virtual machine manager. And Oracle does offer up one extension, USB management, that will likely allow users of the open source version (usually distributed by Linux desktops) to get that previously unavailable capability.
VirtualBox 4.0 is a free download for Windows, Mac, and Linux systems. If you're intrigued and want to get started with VirtualBox and virtual operating systems, try our beginner's guide to virtual machines, or take on the seemingly impossible by running Mac OS X on Windows in VirtualBox.