If you are interested in the math behind an involute gear and how to sketch one,
this online tutorial is a good one to follow.
If you want to draw gears to design and build crazy mechanisms and cut them yourself (you'll want a CNC machine), the guy responsible for Mach3 has been developing
Gearotic Motion, which is a pretty incredible tool. It is free to download and play with, and only $75 if you want to be able to export the gears.