You may have started to see codes like these appearing everywhere, you certainly will if you live in Japan. Called QR codes they are 2-D barcodes designed to be easy to scan. Many modern camera phones have readers that scan the QR code and automatically take you to the website that the code is linked to (there are several iPhone apps that do this).
These codes can be put on billboards, in magazines, on products, anywhere, enabling people to quickly obtain information about the thing tagged with the code, using the browser in their phone.
The QR code above is the one that points to this blog. I think I'll get a T-shirt printed with the code on it.
You can generate your own QR codes using the QR-Code Generator.