The Null Device
Defense contractors in the US have developed a gun-mounted system for recognising enemies. The system consists of a camera mounted on a rifle, attached to a backpack computer which recognises and keeps a database of faces; it can alert the soldier to the same face showing up in a variety of trouble spots (i.e., a troublemaker), or being on a pre-supplied list of bad guys/terrorists/ornery varmints. Perhaps they could even attach it to the gun's trigger, so then all you'd have to do is sweep the gun over a crowd and it'd automagically shoot the right people.
(via Engadget, Boing Boing)