I'd like to point out that while I haven't invented anything quite so fabulous as war-chalking, I did come up with the blogger gang-sign. Hold out your left hand, palm up, then grab your left forearm and make a moue of pain as you massage away invisible RSI cramps -- dude, you're throwing signs!
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.