All borders between operating systems are being erased!
I am pretty amazed by how far has digital technology come, both due to hardware and software advancements. At this point a modern computer with GNU/Linux can run multiple operating systems at the same time without rebooting and with none of them running from within a single window (the way virtualization is usually done).
With VirtualBox, which has a Freedomware version (OSE edition), you can easily run a number of GNU/Linux, UNIX and Windows based operating systems on GNU/Linux, Windows and since the newest version Mac OS X as well. If your machine is strong enough (basically, a good processor with preferably virtualization support and at least a gigabyte of memory (I have 1.5 GB)) this pretty much erases all borders between operating systems, which is especially apparent once you enable the seamless mode.
The seamless mode is described here and it basically frees the guest operating system from the window container and sets its applications loose across the host OS making it appear as if your OS is running both OS's native applications.. natively. Essentially, this is truly like you merged the two OS's in one, except that it is just one running within another.
Pretty impressive I'd say.
Considering that performance is excellent (slow down of the guest OS is practically non-existent here) this immediately becomes a perhaps better option than WINE for those who must run some Windows programs on GNU/Linux (not that I recommend it, choose better Free alternatives instead ).
It is also an excellent way to try new Freedomware OS's which are aplenty and if something runs on one and not another, now you can run it anyway!
Did you try this yet? Which guest OS did you use? How was your experience?