Quietnet is a free/open Python program that uses your computer's speakers to encode text messages as near-ultrasonic chirps that can be received and interpreted by other, nearby computers. Its creator, Kate Murphy, notes, "Warning: May annoy some animals."
Quietnet (Thanks, Sulka!)
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.