Sometimes they're not very good. Sometimes they're just very expensive.

There are a range of different hardware devices available on various plugin boards. And processors have started to include them as well.

Here's a very old (1997?) examination of 3 hardware devices: (http://www.robertnz.net/true_rng.html) and he has some nice information here too: (http://www.robertnz.net/hwrng.htm)

Here's my list of recent reading, not all of it relevant.

(http://csrc.nist.gov/groups/ST/toolkit/rng/documents/nissc-p...)

(http://www.paulm.org/random.html)

(http://www.cryptography.com/public/pdf/IntelRNG.pdf)

Build one yourself: (http://www.labbookpages.co.uk/electronics/hwRNG.html)

Here's a list of devices:

(http://www.westphal-electronic.com/)

(http://www.trng98.se/shop/index.php)

(http://www.comscire.com/)

(http://www.idquantique.com/random-number-generators/products...)

(http://www.letech.jpn.com/)

And someone upthread posted a link to EntropyKey.

Thanks, very helpful.

Though I can't speak to they're adoption in actual hardware, thermal noise generators are apparently a common way to generate random numbers (https://en.wikipedia.org/wiki/Hardware_random_number_generat...). I would be interested to hear details on how the pi's generator works.