Lasers are already being used in the field to shoot down missiles and such, but they are limited in their applications because they require volatile chemical agents or fragile crystal arrays — neither one something you want in a battlefield situation. But the Free Electron Laser uses a “turbocharged stream of electrons” to start the reaction that fires the laser. For whatever reason, this sucker never runs out of ammo and is not affected by atmospheric degradation, because they can choose the exact wavelength of the laser they want to fire. Scary, huh?
The bad news (or good news to some) is that its power is severely limited and until a few years ago, the things could barely warm up leftover pizza, much less knock ICBMs out of the air. A few breakthroughs have increased the output of the laser by about 1000 fold, and they promise to increase it to 10 times as much, bringing it up to “weapons grade.” The future is here, but why is it a high-powered laser and not a clean, renewable source of energy? More (old) pictures and info here.