You can also build an apparently pretty decent white noise source by
amplifying avalance noise in a reverse-biased diode.
http://web.jfet.org/hw-rng.html
As is the case with most random noise sources, you obviously want to
whiten the output before adding it to your entropy pool.
http://en.wikipedia.org/wiki/Hardware_random_number_generator#Software_whitening
-=rsw