Anyone out there have an iphone?
This may not be perfect but it's cheap (like, um, free) and works well enough for me to be happy with the measurements I get. I also find it very repeatable.
Get this app...
Build a jig that holds your phone and the rotor at a set distance from each other. I used a small plastic block, wood will work too, just be sure the rotor is in a fixed groove that it can rotate in and the phone is held in place as well.
Then...measure the rotor + and - by slowly rotating it around while using this app. The app reads to 999 so you need to set the jig up so that you don't go over this point. My first jig read in the 5-600 range for decent rotors and worked fine. You can set the rotor closer to the phone for finer increments.
So far I have $0 invested and I don't feel that I need anything more to test rotors.