QR (Quick Response) codes are a type of matrix barcode, commonly used to store text and links, which can be scanned and read by smartphone apps. They're now appearing just about everywhere, and if you want to join in then Alternate QR Code Generator makes it very easy to create codes of your own.
At a minimum, you can just enter the text you'd like to include, and click "Generate" to see the appropriate code.
There are also a range of options which could help to improve the code's reliability. Increasing pixel width or error correction level, for example, will make the code larger, but also ensure that it can sustain more damage while still being readable. You can change the background and foreground colours, too, which also may be useful in some situations (if the code is being printed on coloured paper, say).