There are plenty of fancy Geiger counters on the market. Care to build your own? This MightyOhm Geiger Counter Kit++ is worth a look. It comes with everything you need to detect beta and gamma radiation on your own. It is open source and hackable.
This DIY Geiger counter comes with a LED and buzzer for feedback upon detection. It also has a serial output for logging. It runs on an ATTiny2313 microcontroller.