Summary

Printable anemometer - clips onto a cell phone (designed for a Nexus 5, could easily be modified to fit a different phone); the phone detects rotations of an added rare earth magnet.
The beginnings of an Android app for reading the meter are here: https://github.com/berkenbu/Anemometer

Instructions

Print one of each:

anemometer_phone_mount
anemometer_top_rotor
anemometer_rotor_bottom
anemometer_magnet_holder

Also required:

magnet - I used a cylindrical, 8mm diameter rare earth magnet from the hardware store
ball bearing - I used a 6mm OD, 3mm ID one from a toy helicopter
axle - I used a 3mm dia machine screw with its head removed

The 'phone_holder' has a cylindrical hole of 6mm diameter that I used to install a small ball bearing (from a broken toy helicopter) - this might need to be adjusted if you have a different bearing.
I used a 3mm machine screw with the head removed and matching nuts and washer (for 'sandwiching' the bearing - the rotor and magnet holder just screw on) as an axle - something else could be improvised...

I split up the main rotor into two parts to be able to print it without supports: the top and three bottom 'cup segments' are to be glued onto the top.

The github for the app is given in the main description - it is very much work in progress. At the point of this writing, it will display RPMs of the rotor (plus a scrolling recent history graph), but not actual wind speed. One would need to calibrate for actual wind speed measurements, possibly with an existing, calibrated anemometer, or a source of known, constant speed wind.
The app was written in Android studio and should compile without additional libraries - there is a pre-compiled apk in the folder 'app'.

Nexus 5 anemometer