QR Codes (Quick Response Codes) are popular. They allow a smart phone to use the camera to scan a graphical code and then be taken to a website or provide other textual or contact information. You will need to download a QR Code Reader app on your smartphone to be able to scan QR Codes. Search for QR Code Reader and download any free one.
This page lists websites and other resources that provide you with useful information about creating and using QR Codes for your marketing purposes and gives great examples of how others are creatively using QR Codes.
See example of a QR Code above.