The Air Force Research Laboratory developed a microwave gun device like this that is intended for use as a crowd deterrent.
The device uses of a 6-foot dish that can be mounted on ground or low-flying air vehicles. The dish is able to focus a beam of 3-mm microwaves on a single target, or sweep them across a crowd. Anyone caught in the beam begin to have their skin heat up to the point where their pain reflex will cause them to involuntarily move out of the path of the beam.
Theoretically, this weapon won't burn people, assuming that the beam isn't aimed at the same place for too long. However, some scientists are still worried that even if the microwaves don't cause skin damage, they still could cause eye damage.