Back in December I posted about a new USB device with dedicated hardware for producing a continuous stream of high entropy bits based on sampling P-N junctions. Another lower tech randomness source with a USB interface is described at this site, and is shown below
The device has a small hourglass, and as the sand falls from the upper to the lower chamber, the pattern of grains is sampled against a light-sensitive detector at a rate of 100 times per second. The site claims that each sample yields about 9 bits of entropy based on statistical tests. The device detects when all the sand has passed to the lower chamber and then rotates the hourglass 180 degrees, so the sampling process can continue. The samples can be accessed through a USB interface.
The device is a prototype, and as yet not for sale, but costs about $100 to produce. The advantage of the hourglass method over other more sophisticated and higher yielding devices is simplicity and transparency. Perhaps so, and you can read more about the design here, and the entropy of the output here. Finally
While the hourglass is not precise, accurate, or repeatable as a timekeeper, and has been almost completely supplanted by better devices, it is a good source of random entropy. It is still manufactured in quantity at low cost, and it is clean, compact, durable, and uses little energy. The source of the random entropy can be easily understood, and observed to be functioning correctly without instruments. An off-the-shelf photointerrupter can be employed to electronically observe the random entropy, and an open-source, standardized microcontroller can be used to control the process and interface it with a host computer.