Perpetual Marble Run.
Most marble runs just have marbles which run from the top to the bottom and then have to be reloaded.
This marble run uses a battery powered electric motor (OK, it isn't perpetual!) and some clever gearing
and levers to bring the marbles back up to the top of the run.
The files for this model, devised by Ubaid Mithawalla, are available from MyMiniFactory for $5.