This a DIY Anemometer designed in FreeCad for a bigger proyect, a weather station (still in progress).
The central part has been designed to be connected by the rotary part with two M5 ball bearings. I selected the 625 ball bearing from the RepRap wiki (http://reprap.org/wiki/Ball_bearing) because they are used in some RepRap printers and can be acquired in most of the 3D printers stores. You will need to print two pieces of this.
The bottom part is designed to hold the magnet and spin at the same speed as the part that catches the wind. This allow me to use a Hall effect sensor to measure the wind speed.
The part with the hemispheres is based in: http://www.thingiverse.com/thing:19439 made by http://www.thingiverse.com/rlittin/about but redesigned in FreeCad and escalated.