Quickly and easily make templates for custom gear systems.
Woodgears offers a handy app that automatically makes templates for gears that will properly mesh based on the size desired, number of teeth, etc. It is intended for wooden gears, but I suspect it will work for other materials as well. The catch is that the free online app only exports prints, not laser-cuttable files. For those so inclined, there are good instructions for making wooden gears using non-digital fabrication.
They also offer a more sophisticated version for $26 with a lot more features that will export DXF and Sketchup files (PC only).