A slight remix of PerryT's wonderful Elliptical Gear set. I liked playing around with the gears connected at the off-center pivots, and thought it would look cool to remove the center ring entirely.
You can find the original thing here, along with other types of connecting arms: http://www.thingiverse.com/thing:37659
I've made a new oval gear with a spiral design in the middle. Check it out here: http://www.thingiverse.com/thing:1125461
Make sure the holes are smooth and pop in the pins gently.
Be aware that the left and right sides of the gear have different teeth positions with respect to the hole. Thus, you may need to rotate one of the gears 180 degrees to get the right alignment. If this doesn't make sense, look at the photos below.
Incorrect Alignment (right gear was rotated 180 degrees)