Since smoke signals are somewhat obsolete in terms of technological advancement, this Lampex USB Magic Wand looks like it could do the trick for sending secret visual messages. Connect the Lampex to your PC, type in the message you want displayed, and then wave the little bugger through the air.
As you swipe it through the air LEDs (or perhaps some magical elf) blink in time to produce your message in midair. Can Harry Potter do this? I think not. Though we doubt this wand would be any help against The Dark Lord either. — Andrew Dobrow