Semacode works by embedding a URL (web address) into a sort of two-dimensional barcode which looks like a dense crossword puzzle (pictured) — called the tag. The SDK software contains the capability to detect and decode the tag very rapidly with the camera on your phone. It extracts the URL and sends you to that address using the phone’s built-in browser. semacode example This may seem complex — and technically, it is — but the user experience is simple to use and easy to learn. By connecting users from physical media to mobile content, the system creates new affordances, new ways to interact with the objects and media that surround us (including augmented reality, remote presence, social software and location tagging). In each field, be it marketing, tourism, games, etc., there are a number of opportunities to use this technology to improve the way people interact and communicate. (from semacode.com)
There are all sorts of projects based on the technology; the most interesting of which I think is Semapedia, which generates these things for Wikipedia URLs that you can then affix to physical objects.
Very cool stuff.