A new version (1.0.1) of the Entropy library is available. You can obtain it from the Google Code site or for convenience you can download this zip file from here Entropy Library version 1.0.2
The new version includes a change from Paul Stoffregen that allows the library to run on his Teensy 3.x boards.
A library for AVR's or Arduino's that make use of the watchdog timer's natural jitter to produce a reliable stream of true random numbers. Unlike other methods currently available, this library requires no external hardware and produces a considerably more random number stream than methods based upon the technique of reading an unconnected analog pin.
Arduino forum. The information on that thread caused me to begin experimenting with the method and finally conclude that it appears to be capable of producing cryptographically useful random numbers at best and the best source of unpredictable numbers on the Arduino platform at worst.
This library only works on those AVR chips that have a watchdog reset interrupt vector, as such this library will not work with an ATmega8 series of chips. This library is a fully function integer based random number generator; however, if you only need to provide a good seed value to the internal random function a short example of how to use the same basic techniques as implemented in the library without the overhead inccurred can be found here.