An optical sensor checks this mini hourglass eventually triggering the rotating mechanism to flip the thing 180 degrees. It also can send the optical sensor's values to a PC by USB, providing random numbers. I just think it looks cool.

There are some good non-deterministic random sources disclosed on the Internet. They make use of radioactive decay, radios tuned to secret frequencies where there is no broadcast, dice rolling down a chute, and even lava lamps.

The USB Hourglass compares well with these sources in terms of bit-rate, cost, safety, reliability, and simplicity.

[USB Hourglass via boingboing]