Eirik Taylor has built a Geiger counter, particle detector that measures ionizing radiation, using microcontroller AVR ATtiny2313 as main controller. The counter could display all sorts of data based on the counts from the Geiger tube on LCD display. The raw count data can be sent directly over a serial connection to a PC, thus allowing for remote monitoring of radiation levels. The count values could then be analyzed later using software and graphed using a spreadsheet program.
The Geiger tubes use an organic quench gas (methanol or isobutane), and as such require a higher operating voltage. If you want to build this project, be sure you can construct a high voltage supply for the tube first, as this is the hardest part of the project. Download Geiger counter project.