This is my new, favorite random number generator (old favorite: lava lamps):
It watches falling sand in an hourglass with an optical sensor. That data is sent via the Arduino USB output to the PC where it’s analyzed. This entropy is useful for all your random number needs. My favorite part: when the hourglass runs dry a servo motor flips it over and it starts again.
Says maker Peter Allan:
With the USB Hourglass, the user can look at the sand falling through the center of the hourglass and monitor the randomness in the USB output data. And one can read the code line-by-line, compile it, and upload it to the microcontroller using only open-source and widely supported tools.
[Thanks, Scott Burris!]