Yesterday, I attended to Enrico's rehearsal presentation about tagging and he made an interesting list of existing visual markers:
 | AR toolkit | |
 | visual codes | |
 | spotcode | |
 | QR code | |
 | Datamatrix | |
 |  | d-touch |
 | mytago | |
Why do I blog this? to have a kind of list of such technology, just in case one ask... there are the visual representations and the links