The concept works by using a pair of LASERs operating at about 193 nanometers. When the weapon is fired, the LASERs ignite and create two ionized particle trails between the weapon and the target. The ionized particle trail has much less resistance than normal air, allowing current to flow towards the target.
This idea is actually nothing new. I had this idea back when I was a junior in high school, and that was without any kind of external references about this specific topic or really even knowing that much about LASERs. Ground Control mentions that he had the idea too.
The major problem in producing these things is getting a LASER diode which puts out enough power and emits light at the correct frequency. I would suspect, however, that these things have been built and are probably being carried by people. For obvious reasons, nobody who created the necessary LASER emitters would publish information about them or sell them because they really only have one purpose, and that would tip the police off that the person probably has one of these. Anyone who had built the actual weapon would best not make it known either, for the same reasons: they don't want the Department of Defense or the police breaking down their door.
It's also interesting to note that if one of these were used, the target would just loose muscular control and then would probably be taken down by more conventional means, meaning that the use of this stun weapon wouldn't be known.