See http://xkcd.com/936/ Basic argument is that picking 4 random common words for a password gives you about 44 bits of entropy, vs, say 28 bits of entropy for a "strong" (but patterned to be kinda memorable) random password. xkcdpwgen (should) let you specify how strong a password you want (either in bits of entropy, or as equivalent to a random string of printable ascii characters of some given length) and generates candidate passwords as a list of words you should concatenate together. It defaults to 44 bits of entropy as per the cartoon.