Random Culture has an interesting post about Quick Response Codes, which are being used extensively in Europe and Japan for mobile marketing campaigns.
These codes, created in Japan in 1994 by Denso-Wave are being used in marketing for mobile users to take a picture of the code or read a the codes with the cameras to trigger action on their mobiles, it could be to download some piece of content, get a message or go to a website.
QR Codes might be placed in magazines, buses, signs, business cards to let users with the special software installed on the mobile to capture the image and executed the desired action.
For more information go to Random Cuture or the Wikipedia.