Have you ever wanted to turn a CD-ROM motor into a winch? I decided to see how much gear reduction could be fit into a small space, so I came up with this differential planetary arrangement. This is a well-known design, but now you can use the parameterized OpenSCAD version to customize this to your needs and include it in your mechanical designs.
The version shown here is less than 60mm in diameter and 13mm thick, but achieves a 1:246 gear reduction, all with teeth printed on a TOM. The input is the small shaft to the sun gear, the output is the smaller ring gear, and the larger ring gear is to be held fixed. If you stacked two of these units you would have a ratio of 1:60516!
Print one each of the three STLs. The sun gear's shaft hangs though the hole in the larger ring gear. The three planets are placed around the sun gear (they have one more tooth on the bottom than the top), and their orientation is important. Each planet has exactly one place where the tooth on top is exactly above a tooth on the bottom. These must all point the same direction when the gears are assembled (i.e. along the x-direction). The way fourgears.stl is made, the planets are all lined up like this, so the fill should help with the orientation (make the fill lines parallel). Finally, place the smaller ring gear on top. It should slide right on if the planets are oriented properly.
The OpenSCAD file is commented and parameterized for easy tweaking. It will output the gear ratio you get based on the number of teeth you use. It is also set up to visualize through animation: just compile the assembly, then select view->animate. I'd recommend setting steps to more than 2000 or else it will be jerky. This file makes use of Greg's involute gear script, and it's called through the MCAD library.