This 64:1 gearbox is inspired by the Tamiya gearboxes, which are nearly all-plastic construction but work great all the same. This hasn't been printed out before.
For use with the Mabuchi DC motors:
There is a 3D model of this motor in the .blend file.
Edit! I'm getting reports of gears not meshing at this size from the MakerBot team, and scaling up this design 'till they do puts the frame outside the bounding box. I've got some ideas, though, so hang in there. Meanwhile, anyone with good enough resolution to get gears to mesh at this size (or 2x for that matter) should still be able to get this working.
Notes on 2.0: Despite this thing being mechanically identical in nearly every way to the previous versions, version 2.0 has been re-built in Blender from the ground up. More importantly, a clip has been added to connect a DC motor to the gearbox. The above link should take you to a site where you can buy them for sixty cents each!
For archival purposes, version 1.6 has been left in as a .zip archive.
Patch! 1.6: The new files have been patched so that all of them slice in Skeinforge without errors or warnings, making this the first version I'd recommend printing without going "eeennnnhhh..." after doing so. The horns also got a nice upgrade which gives them attractive rounded-off corners. Still don't have a motor clip though....
Notes 1.5: After uploading Gearbox v.1.0, I decided to take a look at skeinforge and the Blender-STL conversion to try to make some predictions about how manufacturable this thing is. The good news was pretty good: I had roughly the right scale (Assuming those toolpaths for the gear teeth work out okay) and there were no non-solid objects! (I was worried there were tiny holes in the gears.) The bad news was that I could see a number of potential mechanical troubles: the interface pins would print out as tiny "nubs" if at all, and the pins didn't do a lot to keep the gears from wobbling.
In addition to fixing the more obvious flaws, I added a proper set of "servo horns" that can be printed out along with the main gearbox, as well as a pair of slots on the bottom plate for the user to attach a motor clip of their own making. In version 2.0 (an if/when sort of thing) there'll be a specialized motor clip designed to snap into the slots, along with a collar for the motor in the bottom plate.
Print all parts, print a second copy of the 4:1 gear, and assemble according to the diagram. Glue may be needed to hold the corner posts on, as well as the horns. Lubricant's probably a good idea.