The smaller and undetectable the spying drone, the better.  British forces have been using the PD-100 Black Hornet for reconnaissance since last year. The whole thing weighs just 16 grams.

Developed by Norwegian-based Prox Dynamics AS, the Black Hornet can fly for up to 25 minutes on a single charge. It can cover about three-quarters of a mile at its top speed and it uses GPS to fly from waypoint to waypoint on its pre-defined route.

The Black Hornet is used to gather footage during daylight missions. Its creators claim that it is completely invisible and silent at distances over 30 feet.[Prox Dynamics AS via LiveScience]