This LEGO Technic-compatible double bevel gear has a rounded central hole so that it doesn't necessarily rotate with an axle passing through it.
Among other things, this allows for custom differentials to be built (high torque open differential, limited slip differential, ...), while using less space than a turntable.
This gear has a rather trivial placement of holes. A variation of this gear with a special disposition of holes, which can be used as a planetary gears carrier together with my ring 40-teeth gear
, is also available here