I love xkcd.
This one, about password strength applies to permutations and combinations.
Dan Anderson describes a lesson that could be built around this cartoon. Have students use permutations and combinations to play around with password strength. I’d use this applet that Dan has screenshots from in his post to let students test various ones and check the math.
If you like this stuff as much as I do, I have to also point out that dropbox picked up on this one.