This uses the spreadsheet from http://www.thingiverse.com/thing:7881 as a design table for a Solidworks part. Tested from 3 to 100 teeth at pressure angles of 14.5, 20 and 25deg. Currently only does straight spur gears, but it's really easy in Sldwrks to make an internal or helical gear.

I realize only a minority of people on this site use Solidworks. I use it a lot, and I'm all that matters in the end :)

Open the part and edit the design table where you can change the module, number of teeth, pressure angle and profile offset (first 4 properties in the table). You can mess with more properties (addendum, dedendum, etc.) in the 'Calc' tab in the design table.