Holy crap, you guys, bomb proof wallpaper. This stuff is strong enough to keep very heavy flying objects from breaking walls—in this video, a wrecking ball.
The X-Flex wallpaper is really a layer of Kevlar-type material, in between sheets of "elastic polymer wrap," which provides both flex and strength so that the projectile doesn't knock the wall down. It seems really effective—check out Pop Sci's video here for evidence—and the US Army is considering using it for bases in Iraq and Afghanistan. It's also incredibly easy to attach, being basically a rollable sheet, although I'm not sure if it can be unattached and reused. Still, its effectiveness is really impressive. [Pop Sci]