The best thing is that Extra Reality provide an SDK to develop your own AR models, which can then be uploaded to their servers to be identified by clients worldwide. And it's free for non-commercial use.
I'm a bit worried about their marker code, though, as it seems to contain a very limited number of bits. If Popcode becomes hugely successful (and I really hope so), they'll have to come up with another scheme.
Sadly, it's only available for Android right now, so I can't really test it (hey, benevolent sponsors to be, this is a call for help!).
See more demos at Popcode's website.