We all know about standard types of gearboxes. But, did you hear about Cyclo Speed Reducer or Cyclo Drive Gearbox? The main element is cycloidal gear you can see at the picture below. They have the greatest application in robotics.
Their main feature is the transmission ratio. I designed a Double Cyclo Speed Reducer whose gear ratio is 1:121. You can find this speed reducer with all data in the following link:
What do you think about this kind of speed reducer and CAD model?