Improvise a Hardware Random-Number Generator

Shell

Reads a single sample from the microphone stream and prints its least-significant bit, which should be dominated by noise.

EDIT: Changed to unmute the microphone ... and everything else too!

# Warning - unmutes EVERYTHING!
for DEV in `amixer | grep Simple | sed -e "s/.*'\(.*\)'.*/\\1/" -e "s/ /~/g"`
do
    amixer -q -- sset "`echo $DEV | sed 's/~/ /g'`" unmute 100% 2>/dev/null
done

echo $(( `od -N1 -d < /dev/dsp | head -n1 | sed 's/.* //'` & 1 ))

Bash

echo $[`ping -qc1 127.1|sed 's/[^1-9]/+/g'`0&1]

Gathers entropy from the response time of a single ping to localhost.

Note that the response time appears exactly thrice in the output of ping -qc1:

PING 127.1 (127.0.0.1) 56(84) bytes of data.

--- 127.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.044/0.044/0.044/0.000 ms

All other numbers and constant and – more importantly – independent from the response time.

sed 's/[^1-9]/+/g' converts every zero and non-digit into plus signs, and echo $[...0&1] prints the parity of the resulting sum.

JavaScript + HTML5 DeviceMotion

var hash = function(x) {
    var h = 0
    for (var i = 0; i < x.length; i++) {
        h += x.charCodeAt(i)
        h ^= h << 5
        h ^= h >> 3
        h ^= h << 13
        h &= 0xffff
    }
    return h
}
var listener = function(e) {
    var accelerationString = JSON.stringify(e.acceleration)
    var hashed = hash(accelerationString)
    alert(hashed % 2)
    window.removeEventListener("devicemotion", listener, true)
}
window.addEventListener("devicemotion", listener, true);

JSFiddle here.

Uses the HTML5 DeviceMotion API on supported devices (mostly mobile devices). It turns the resulting acceleration object into JSON, hashes it, and takes the remainder modulo 2.

Most of the code is the hash function (damn you JavaScript, and your total lack of a standard library). It could probably be shorter, but I'm a sucker for a good hash function.