I adapted stickoutrock's Crosslink Ellipse Gears to be printable on a 3D printer. It's similar to natetrue's version ( http://www.thingiverse.com/thing:1538 ), but I created a completely new design in OpenSCAD, which uses a different type of joint, which should be more robust. The joints also do not stick out of the holes in the ellipse gear, avoiding the gears to get stuck during the rotation.

Here is a video of this thing: http://vimeo.com/18244936

You need 2 of the gears and 2 links. The gears are NOT symmetrical. Before snapping them together, make sure you have the gears aligned correctly: Place them on top of each other as shown in the dxf preview and make sure they have the same orientation. Move one of them sideways until the teeth mate.