I saw this post a while ago on the mach1mach2cnc yahoo news group. Art Fenerty, who I believe is the original person behind the Mach3 PC Based CNC Controller, posted a little teaser video of his gear designer software in action. He calls it GearoticMotion in his post.
The software is not limited to simple gears. It can do non-circular gears and many other interesting shapes. It animates the whole mechanism in 2D or 3D. I love the fast way you can place the gears, where it constrains the axis of the next gear being placed to the correct distance from the meshing gear. It will output files ready for CNC cutting.
It is still a few months away with an estimated price of $75.