A small vertical-axis wind turbine (VAWT) which fits on the top of a pencil.
It's not very efficient, but it does work!
Good as a physics demonstration, or for just looking nice in front of a desk fan.
The fit of the center hole is close enough that it works with some, but not all pencils! If you want a looser fit, scale the model accordingly.
The print pictured was made with supports, but it should be printable without.
This model uses the excellent Naca4 library (thing:12634).
Drop Naca4.scad in the same directory as this model in order to view it in OpenSCAD.