From IEEE Spectrum,a "cyclogyro", or "cyclocopter":
Fundamentally, a cyclocopter is similar to a helicopter in that it creates lift through rapidly moving airfoils. Unlike a helicopter, a cyclocopter's airfoils rotate around ahorizontal axis, continually changing their pitch in order to generate thrust in one single direction:
It's certainly not a simple system, which is why this idea (which has been around in the form of various prototypes for nearly a century) only got off the ground to make a first untethered flight just recently, thanks to a lot of hard work from Moble Benedict and his team at the University of Maryland. They've been developing a cycloidal rotor system made of carbon fiber and titanium that's so far been applied to both a quad cyclocopter and a twin cyclocopter, and they've successfully gotten the two rotor version (with a supplemental tail rotor) into an untethered and more or less stable hover.
You're probably wondering what the advantages of such a complex system are, and luckily, there are a few. Primarily, it's suggested that a cyclocopter would be more efficient than a helicopter, able to generate more thrust for a given amount of power. It's also thought that cyclocopters will prove to be more maneuverable, since the thrust can be vectored very rapidly. On the downside, you've got the overall complexity of the system to deal with, and the weight of the rotors might cancel out any efficiency gains.
Paper (pdf) here. Video of earlier demo: