Audio Entropyd
Audio Entropyd is a small program that I wrote to read data from a soundcard, hash it and feed the result to the Linux kernel's random number pool. Since this audio contains at least small amounts of noise, this may be a cheap source of entropy. audio-entropyd takes the difference between stereo channels in an attempt to eliminate hum.
2005-05-05: This code has been relicensed to a ISC/BSD copyright.
NB. My version is long since unmaintained, but Folkert van Heusden and Udo van den Heuvel have an updated version available here.
Caveat Emptor
Since part of the motivation to write audio-entropyd was to silence a long and hypothetical thread on a crypto mailing list, it has had little use and testing. In particular, there has been no work done to estimate the entropy rate of the data it generates. Please do not rely on this software for security.
This disclaimer applies to my original 0.0.0 version of audio-entropyd only, I have not examined and therefore have no opinion on the revised version
Download
If you, for some unknown reason, desire my original version (and not the updated version mentioned above), it is still available: