QR Codes (Quick Response Codes) are two dimensional bar codes, developed back in 1994 by Denso-Wave, and are currently very popular in Japan, mostly used for marketing.
With more mobile phones are now equipped with a reasonably good digital camera, QR Codes are used for storing addresses and URLs (known as mobile tagging), allowing mobile phone users to easily and virtually error free bookmark and/or launch URLs in a mobile browser.
While probably more for fun than actually usefulness, this blog now contains QR Codes at the end of every blog entry - well, in the printed version, there may really be a good use-case for including a QR-Code. Anyway, there are a couple of ways how to dynamically generate the QR Code images on the fly.