For the chumps (like me) who don't know how to get the perl mcrypt module:
sort -R /usr/share/dict/words | head -n 4 sort -R /usr/share/dict/words | head -n 4 shuf -n 4 /usr/share/dict/words
Somewhat more seriously, Diceware works pretty well too as a low-tech, high-quality password generation method: http://world.std.com/~reinhold/diceware.html
(My only complaint is that a lot of words in the standard wordlist are pretty obscure.)we wrote this one at my company. It uses your mouse movements as a seed for the random number generator.