Pretty cool thing to see while being irradiated.
A couple of years ago I bought a Geiger counter from Sparkfun. It works pretty well albeit somewhat boring; a blinking light and an ascii character via its serial port. Then one day I had an idea: I should make it visually more attractive. I had tons of led-matrixes laying around (they cost almost zero china dollars) so I soldered an arduino to the signal-pin of the geiger counter and created a "sketch" that switches a led on or off depending on if it saw a pulse in the current second. Others may have a different view but I find the result dramatically interesting; this is movie material!
Source: PimpMyGeiger via Hackaday
Learn more at: http://paxinstruments.com/pimp-my-geiger-counter/