http://xkcd.com/1150/
I'm not a big fan of xkcd but when they nail it, they nail it.
Remember: If You're Not Paying for It; You're the Product
That's a good one.
I don't know if I'm a fan. I only go there when someone links, but he's pretty amazing (no "they" just Randall Munroe). Like you say, when he nails it, he nails it.
I was trying to write a report for a client that, among other things, touched on password security. I couldn't think of any better way to explain it than
http://xkcd.com/936/
http://xkcd.com/792/
Other than something I sent to NFFC to hang in his office (not xkcd), this is the only drawing or art I've ever purchased off the internet
http://xkcd.com/386/