Sounds great! And great to see there's a mac version.

I use a similar concept for MacOSX but mine is not freeware. Thus I have Windows 2000, 7, 8 and two XP's. I'm yet to get a Linux to try out, something I should brave to do. But I'm not much of a terminal user. And I'm not sure which Linux would be best to try. I mostly only use these for testing, but do play the odd game.

There are a number of virtual softwares around. Apparently with some, you can open a particular virtual machine with other equivalent virtual softwares, not sure that's possible with Virtual Box.

You said virtual box is a totally closed environment. Does that mean you cannot copy files to or from it or connect to the internet? Such abilities are totally customisable with mine. If you want or don't want any interaction between and to what degree.