We’re no stranger to radiation detector builds, but [Dmytry]‘s MicroGeiger prototype is one of the smallest and most useful we’ve seen.
The idea behind the MicroGeiger comes from the observation that just about every modern smartphone can provide a small bit of power through the microphone jack. Usually this is used for a microphone, but with the right circuit it can be stepped up enough to power a Geiger tube.
Here's my page about the project, describing the operation in greater detail. Also, the hand would transformer can now be substituted for with a very available alternative: an inductor from a broken compact fluorescent lightbulb, with a couple small extra windings added to it.
A few photos: