They've just officially made the first test apparently, this thing is capable of destroying ICBM's in the sky (in the early booster stages of launch, so I've heard). Research the rest for an in-depth look, as right now I'm ignorant and currently reading up.
http://archives.cnn.com/2001/US/01/12/airborne.laser/index.html
http://archives.cnn.com/2001/US/01/12/airborne.laser/index.html