Catapult

The first version is a floating widget catapult (those patented plastic balls that you find in some black cans of beer).
Watch it in action:
http://vimeo.com/29575935
 
The second is an OpenSCAD catapult with one single parameter.
Just enter the diameter of your projectile and let it build arm and base accordingly.
Short animation showing variation of "projectile_diameter" from 10 to 35 millimeters:
http://vimeo.com/29589013
If you want a longer arm catapult also adjust the "catapult_length" parameter.
 
Floating widget image from:
http://en.wikipedia.org/wiki/Floating_Widget