24th

Two DIY Geiger counters using smartphone.
Radiation-watch.org produces a low cost ($46) sensor from photodiodes and foil that plugs into the headphone socket. pachube helps aggregrate citizens’ readings to crowdsource radiation maps such as Japan Geigermap. (via One Per Cent: DIY Geiger counter smartphone app to measure radiation)
RadioactivityCounter is another smartphone Geiger counter app for Android and iOS that uses the onboard CMOS sensor and not external sensor to register gamma rays (and a beta on some phones).