Holy crap, you guys, bomb-proof wallpaper. This stuff is strong enough to keep very heavy flying objects from breaking walls — in tests, one thin layer was enough to keep a wrecking ball from knocking down a concrete wall.
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]