QR Codes are 2D barcodes designed to give complex information in a compact and efficient graphical format.
Use your camera phone to snap the code and instantly get contact details, website URLs and more.
Read Wikipedia's
QR Code article for more information.
Many phones already come with a barcode reader. If your camera phone doesn't, I recommend
Kaywa Reader (mobile friendly site)
I-Nigma (mobile friendly site)
Neo Reader (mobile friendly site)
Google's Zebra Crossing (ZXing) (mobile friendly site)
QuickMark (non-mobile site).