Getting a truly random number in Arduino
The Entropy library uses:


  the watchdog timer's natural jitter to produce a reliable stream of true random numbers


I like this solution because it doesn't use up any pins of your microcontroller and doesn't require any external circuitry. This also makes it less subject to external failures.

In addition to a library, they also provide a sketch that demonstrates the use of the same technique used to generate a random seed for the microcontroller's PRNG without the library:
https://sites.google.com/site/astudyofentropy/project-definition/timer-jitter-entropy-sources/entropy-library/arduino-random-seed