Semaphore   

Semaphore is a signalling system which involves waving a pair of hand-held
flags in various positions to indicate letters of the alphabet or numerals.
The flags used in modern semaphore are usually square, red and yellow and divided
diagonaly with the red part on the top left. Semaphore in various forms has been used
to send messages over distances since at least Roman times.

Links

Free semaphore graphics
http://www.nwsc.org/graphics/semaphores.shtml

Text to semaphore convertors
http://www.scouting.org/fun/semaphore/
http://wxs.ca/applets/semaphore/ (animated Java applet, slow to load)