VirutalBox 3.2 Beta 1 was recently released with a new feature that will allow you to virtualize OS X. Right now the virtualized OS X will only run on Apple hardware but there is potential to get it running on Windows according to Life Hacker.
Typically a user running OS X would run a virtual Windows environment to access programs and features not available in OS X. Now a user could eventually run Windows with a virtual instance of OS X available to run programs or features that Windows lacks.
Along with the added support for an OS X guest the name has changed to Oracle VirtualBox since Sun Microsystems was bought out by Oracle in January of this year. They have also added multi-monitor support so you can take advantage of multiple monitors in your guest operating system. Along with pretty extensive list of major and minor updates and bug fixes.
Image courtesy of the VirtualBox Forums