It's good to know a radiation level in your neighborhood, but carrying Geiger counter around is a little bit uncomfortable and can attract unwanted attention. ;) With this things in mind I decided to build my own Ethernet enabled Geiger counter. I used old STS-5 detector tube, manufactured few decades ago in Soviet Union. Voltage converter is supplying tube with 400V, needed for its normal operation. Pulses generated by the tube are amplified by opamp and then counted by Atmega 328 microcontroller. Communication with device is based on a popular ENC28J60 controller chip and simple TCP/IP stack from tuxgraphics.org. Measurements can be accessed by sending UDP packet with a proper request.
Device is also equipped with two additional sensors: DHT11 (temperature and humidity) and BMP085 (temperature and pressure).
I made PCB for this project myself, using thermal transfer method. Etched traces were tinned with Lichtenberg's alloy (great method, worth of recommendation). PCB was designed to be installed inside of standard DIN rail enclosure, which is easy to mount on a wall and has thick gaps on top and bottom side. Thanks to that there is an air flow forced by stack effect - air warmed by electronic components rises, making room for colder air which is sucked from outside, passing sensors on its way.
I am using simple PHP script to read sensor from a web browser. There is also a simple deamon running on my Raspberry Pi, which periodically reads radiation level. If measured value will rise above certain level, program will send an e-mail notification to me.