Conic spiral guide spool. (For lack of a better description.)

This might have use for pull-string applications where you want a bit more torque to get things moving, but more speed at the end. Little guide lip of the spiral shape keeps the string from sliding down the cone as it's pulled.

Of course there may be other uses, but that is one I can think of.

---Edit--- Added a second variant, based on the "Extra objects addon" logarithmic spiral. Modeled a bit different than the first one, but serving a similar purpose.