Gears are an interesting set of things to print. With gears you can make all kinds of things, and if you run an Arduino or a Raspberry Pi, you can significantly extend the toolbox when you can design custom gears. Of course, on Thingiverse, you have literally thousands of gears to pick and print from, but surely you want to make your own?
Blender itself has a Gears add-on, which you can install merely by downloading it and using the Install Add-on feature in the User Preferences. It adds a new type of meshes to the Add Mesh menu, namely Gears. In it you have Gear and Worm Gear, of which I will leave the Worm Gear for later.