Can Spy On:
Anybody with the brains to search for electronic bugs, but not bright enough to bring a flyswatter to squish actual bugs.
How It Does It:
Humanity is united in our disgust and contempt for bugs. They're vile little bastards. Unless of course you work for the government's uber-nerd collective, DARPA, and they're suddenly your best friends, because they make great robots. Why?
One, bugs breed a lot, as anybody who's had them as houseguests can tell you. Two, they have simple nervous systems and aren't cuddly, so nobody cares if you rip out their brains and replace them with microchips just to see what happens, which is exactly what DARPA researchers did.
It turns out that installing a microchip into the brain of an insect is not only easy, it gives you full control over its ability to do things that are great for surveillance, like fly and cling to walls. Attach a camera or a microphone to the bug and suddenly you've got a small, unobtrusive, highly mobile listening device that eats shoe polish and is cheap to replace if it gets crushed.
But we're pretty sure that was just an afterthought between "Let's see if we can rip out a bug's brain and install a microchip!" and "Hey, let's see if we can make them nuclear powered!" Yes, DARPA wants to give us nuclear cyborg cockroach spies. They use a tiny amount of a radioactive isotope to work as a nuclear battery to power the transmitter to relay back to headquarters what you're saying or doing.
We have no knowledge of whether these have been actually deployed by intelligence agencies, which should be a relief because if they were they would totally tell us.