Apparently its new. They just open sourced their product this January 15, 2007. Its similar to Xen and VMWARE. They basically are the same except that It doesn't have the virtual appliance infrastructure available in VMWARE. Am not really sure if VMWARE is truly open source but some of it is it think. Am not really sure which is easier to install though but Virtual Box has pre-built debs under limited license. I think VMWARE has more advantages in the sense that out of the OSes you install on VMWARE Sever you can create Virtual Appliances that you can somewhat mount and unmount to put it simply via VMPLAYER to save you from installing the said OS again. Just mount it and your good to go to. With Xen am not familar with, it but it also has its advantages i suppose. Virtualization technology is resource hungry, you have to have a powerful machine and lots of memory to fill both the requirements of the Host OS and guest OS. If you just plan to play games on it will be somewhat slow and since it the Guest OS must work with the Host OS, you'll be limited to to whatever works in the Host OS causing limited OS functionality. If you want full functionality, you're better off dual booting 2 operating systems. Just my 2 cents anyways, but do try it and tell us how it goes. :)
Cheers!
GaryBrlow :P