Two Disc Roller
This is a fun little gizmo.
It looks like it shouldn't roll, but it rolls very well.
This video show one in action and give a little bit of the physics behind it in the description: http://youtu.be/TgEjxpkzc6s
This is also one of those objects where it is kind of addictive to scroll through the build layers and watch the shape of the build surface change.
I did this as a modeling exercise to see how sculptural and perfect I could get the form and surfaces. I like how the main surface snakes smoothly around the entire form. It reminds me a little of a Moebius strip.
It might even look good as a pendant...