An eccentricly-shaped rotor that by turning (often on a camshaft, though you see cams in drum brakes as well) converts rotational movement to linear movement. Cams look kind of like an extruded cross-section of an egg. I think they're pretty.

Actually, that is an oversimplification. Cams are, indeed, eccentrically-shaped rotors, but their shapes vary according to their purpose. The Clock of the Long Now uses an "Equation of Time Cam" to mechanically convert local solar time to absolute time. It is absolutely a work of art -- a sinusoid twist, smooth and solid and eternal.