(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…