It is well-known that Quadrotor are the most stable form-factor for flying robots, but a swarm of nano-quadrotor flying synchronously just beats every other demo that we have seen. This is the work from the GRASP Laboratory at U Penn, these tiny robots that fit in the palm of your hand can fly autonomously in solo, or in formation (GRASP stands for General Robotics, Automation, Sensing, and Perception). They can even avoid simulated obstacles, like Windows. It is not clear what the flight control system is, but I found their flight to be particularly stable.
Finally, one of the advantage of having small robots is that they may end-up being very cheap to make, and it would basically not matter if they are destroyed or lost. In the meantime, this is an amazing demo to watch. What would you do with them?