Even though there are lots of great gears on thingiverse, I couldn't resist the urge to model some too :)

The combinations of parameters are too many to upload an STL for each, so I just picked a few.

I also made a version for helical gears: http://www.thingiverse.com/thing:1339

The openSCAD file accepts parameters for height, pitch diameter, shaft diameter, teeth (number of teeth, addendum, dedendum, tooth width), shape (solid, star, circles), extrude in and out (diameter, height), rome (pitch diameter, gear height, number of teeth, addendum, dedendum, tooth width, rotation).

There is also an option to involute based on pressure angle, but it's not the real thing - I'm waiting for the rest of the http://openscad.org documentation to apply the proper geometry formula here.