This is a 94.5 : 1 gearbox that is entirely preassembled and requires no support to print. It is a type of harmonic drive, which uses a flexible output ring to attain a high gear reduction. My design is unique (as far as I know) in that it also incorporates a planetary gearset for further reduction. Instead of a toothed belt for the flexible coupling, I use a crinkled shape that forms involute gear teeth on both the inside and outside. The edges of the teeth taper down in order to keep the belt from stretching. The herringbone teeth keep all the gears from falling out.
You can see a description and demo here: http://youtu.be/vzc53n0ssik
Like many of my designs, this is a true torture test of both slicers and printers. I actually delayed finishing this design until MakerWare was able to slice it reasonably well. Still, it's not for the faint of heart. The tolerance needs to be very small to keep it from falling apart, but that often causes the gears to stick together. The flexible ring is weak and may try to break when unsticking the gears from it. Be patient and it may yet work.
You can also use Customizer to make different gear ratios and adjust the tolerance and width to make it print better on your printer.
UPDATE: Nevermind, you can't use Customizer, because they haven't updated to the most recent release of OpenSCAD, which means this design doesn't get built correctly. You'll just have to download the .scad file and adjust the parameters in OpenSCAD directly.