Microsoft has released its answer to QR Codes and Data Matrixes with the appropriately named Microsoft Tag.
Using colours and shape-orientation, the tags are designed to display more information in a smaller place, as well as work when out of focus, which works a lot better with mobile devices where the focal distance is fixed.
Readers for many mobile phones (including the iPhone!) have been released and can be downloaded by browsing to http://gettag.mobi on your mobile device.
You can read more, and make your own tags at http://www.microsoft.com/tag/
I was very impressed with how well it worked on my iPhone. I’ve tried lots of QR/Data Matrix readers out but the poor quality of the camera in the iPhone has always let me down. The Tag worked without a hitch.
They’re still in beta, so no word on if there will eventually be a cost to use these.