I’ve just tried VirtualBox and it simply rocks. I was using VMWare Player and got sick the lack of features. Yes, there’s VMWare Server, but it lacks desktop friendliness. VirtualBox provides the best of both worlds.
(For those not familiar, VirtualBox is a virtualization software: it allows you to create virtual machines, so you can, for example, run Ubuntu inside Windows as a “normal” program, or run Windows inside Ubuntu, or whatever.)
Its “Seamless mode” is very nice: it allows you to treat windows in the guest OS like they were windows in your host OS. (VMWare Player has a “Unity mode” which should do the same thing, but I couldn’t get it to work).
It’s incredibly easy to use, allows you to create your own virtual machines, easily install “Guest Addition” (the equivalent of VMWare Tools, which is a pain to install), mount iso images, use USB drives, etc.
The only downsides I could notice so far is that it’s very slow to suspend/resume and its seamless mode is not that seamless: for example, if you maximize a window after entering seamless mode, the growth area will be invisible; you have to maximize it prior to changing to seamless. Also, Alt+Tab is not “seamless”; it would be so nice if you could switch between host and guest applications with it! It would be probably hard to implement, though…