It’s not as versatile as a true Geiger Counter (it doesn’t measure as broad a spectrum) but for a free application that could be quickly deployed during an emergency to the hundreds of millions of smartphones floating around, it’s pretty awesome. From the app author’s web site:
This App is not one of those numerous “fun apps”, but a true, really working radiation detector. We already did measurements for certain devices using professional equipment at the Helmholtz Research Center Munich, the scale we used for testing starts at several 2-10 µGy/h up to 10 Gy/h (about 10Sv/h) with the latest measurements
Want to play around with it? Hit up the link below to grab a free copy. [Ed. Oops! The app is actually $4.99 for the full version and $1.49 for the lite version.]
Radiactivity Counter [via Hack A Day]