The Real American Warriors - R.A.W. vinyl cling window decal is pretty much the coolest thing around. That is in fact what I heard from someone somewhere at some point in time. It's now on the internet, and thus it is fact.
These babies are die cut, laser printed on a flexible vinyl film that clings to the surface of a window with ease. They can be peeled off and re-stuck multiple times until you get just the right look you're going for. Either side of the decal can cling to the window as well, so it can be displayed on the inside or outside of the window.
I couldn't tell you how it works, other than to say it's nothing short of magic. But who cares how it works... These are pretty much the coolest things around. Remember, the internet said so, no arguing with that.