Safecast Geiger Counter Reference Design
Should we have a Geiger Counter Module for Gadgeteer?

A Geiger Counter Module is definitely on my ‘buy’ list, if only someone would make one.

Uses include the obvious radiation measurements (might want to see or even ThingSpeak is interesting ) but things like random number generators can use these as well.

Hopefully a Geiger Counter Module is coming soon.
Which one is easy to connect to my domido?

what outputs do they have? I’d suggest you look at what options there are for outputs and then pick one you are comfortable in interfacing with your Fez. Personally, I have heard Jeff from Mighty Ohm speak about his kit,, and from the looks of things he has a serial out and a pulse out option, both would be easy to take as inputs into your Fez.