11/17/18: UPDATE: Now saves to STL format. The latest version of GearDXF by Scott at forestmoon.com is 18.104.22.168 and it will now export directly to an STL file! That means you can configure a rack and pinion gear set in minutes and go directly to a 3D printer. Here's a pointer, "The Module (Diametrical Pitch) and the Pressure Angle must match for the gear and Rack, to mesh correctly". Thank you Scott!
11/12/18: UPDATE: Latest version saves in DXF or SVG (laser) format. Currently, if you need to go to an STL format for 3D printing, you can upload the SVG into TinkerCad and save as an STL. GearDXF is an amazing program for rapidly making custom Rack and Pinon gear sets!
GEARDXF is currently a FREE software from Forest Moon Productions http://www.forestmoon.com/software/GearDXF/
I found this software after years of creating Rack and Pinion gear sets the hard way.
With this software you can generate quick custom Rack and Pinion gear sets, in minutes.
Why is this important? When prototyping experimental Laboratory equipment we usually incorporate Robotic components for linear actuation. These most often use Rack and Pinion gears.
With this program you can design a custom gear and matching pinion in about a minute, then import and modify the DXF files using Inkscape (free from https://inkscape.org/ ) and save it as an SVG file for laser cutting. The example for this page designed a rack and Pinion gear (30 seconds), modified it in Inkscape to include and end stop (60 seconds) and it took 1 minute and 12 seconds to laser cut using a Glowforge from 3mm composite board.
It worked perfectly, first time, and the laser kurf even made the fit better. With this approach I can prototype linear actuators in a fraction of the time.
Sincere thanks to Forrest Moon for contributing this to the Maker community!