This design is inspired by old windmills that were used to ground wheat into flour. I always thought they looked quite scenic and was surprised when I couldn't find one on thingiverse.

This was designed in Autodesk Inventor. I went through several iterations trying to find a good gear ratio until I realized two of the same sized gears would obviously mesh well. The pictures and video only show the top two layers and a different gear ratio.

Here are some videos:
https://youtu.be/sdCS0PC_TcQ
https://youtu.be/cmogc6ig7o8
As you can see, the gears don't mesh perfectly so there is some friction. Once the top is removed however, it spins quite fast. The model I uploaded shouldn't have this problem.

Printed with PLA on the Replicator dual on a glass plate.

The blades are the tricky part. I recommend putting all 4 blades together so that the smaller sections have time to cool. Use these settings for the gears too.
Extruder: 200 degrees Celsius
Plate: 60 degrees Celsius
Resolution: 0.3 mm
Shells: 1
Infill: 30%
Rafts: No
Supports: No

Everything else:
Extruder: 215 degrees Celsius
Plate: 60 degrees Celsius
Resolution: 0.25 mm
Shells: 2
Infill: 15%
Rafts: no
Supports: No

Assembly is pretty straightforward. Check the windmill 2.stl in thingiverse if there are any problems.