A QR Code is a matrix code (or two-dimensional bar code) created by Japanese corporation Denso-Wave in 1994. The "QR" is derived from "Quick Response", as the creator intended the code to allow its contents to be decoded at high speed. QR Codes are common in Japan, where they are currently the most popular type of two dimensional codes. Moreover, most current Japanese mobile phones can read this code with their camera. - Wikipedia

On iPhone: Go to iTunes Store here, download and install the 'Barcodes' application to your iPhone.

On Android (G1) Phones: Go to the Android Market and find the 'Barcode Scanner' application. Download and install it.

Windows Mobile and Symbian users: Use QuickMark for these types of phones. It can be found here.

More information is available about QR codes & others types of application used to decode them here.