Originally designed by Toyota Japan for factory use, QR codes are 2-dimensional barcodes commonly used in advertising.
cnn.com: 23 of the coolest QR codes
cnn.com: Does anybody actually use QR codes?
Perl Module GD::Barcode::QRcode
Successful QR Code Campaigns
msn.com: How QR codes hide privacy, security risks
theregister.co.uk: QR Codes and Spam
QR Codes For Memorials
Italian Anti-HIV Campaign Uses Toilet Paper With QR Codes
What Killed the QR Code?
The rise of the QR code and how it has forever changed China’s social habits