Spur and Helical: offset or planetary arrangements, which can be analyzed either as speed reducers or speed increasers. Either Rack or Shaper cutters, with crowning and profile modification up to 4th order.
Double-helical/Herringbone gears are helical gears with opposed helix angles on each half, thereby eliminating axial reactions.
Beveloid gears are spur or helical gears with intersecting axes. One of the members being cylindrical, the other conical, they are insensitive to relative axial displacement