Random number with a cookie memory.

random-cookie-memory

<script>
var classes = ['set1','set2', 'set3'], //add as many classes as u want
  cookieKey = 'previousBackgroundSet',
  getRandomSet = function() {
		return Math.floor(Math.random()*classes.length)
	},
	randomnumber = getRandomSet();

/* Keep generating random number until it differs from the previous */
while(randomnumber == $.cookie(cookieKey)) {
	randomnumber = getRandomSet();
}

/* Set cookie for next time */
$.cookie(cookieKey, randomnumber);

$('div.bg').addClass(classes[randomnumber]);

</script>