Apparently an insect species (Issus coleoptratus) had developed functional gears long before Homo sapiens even conceived of such simple machines. Surprising Science reports the news of the discovery by biologists, Malcolm Burrows and Gregory Sutton. With the use of electron microscopy and high speed video capture, they found that juveniles of these specific planthoppers use this gearing system to coordinate the motion of their hopping limbs, and thus effectively propelling them straight forwards.
Check out this gif from the Smithsonian!
Seriously, read the article at the Smithsonian which goes into further detail of how the gears work, how the scientists confirmed it, possibly why these insects have them, and what happens when these juveniles grow into adults!
Update: More coverage via the always awesome Ed Yong]