It's interesting that this topic should come up today (or rather, it's interesting that I first saw it today). Coming to work there was a giant QR code in my subway car. Just the QR code, nothing else. I needed to know what it was so I snapped a photo of it (or rather scanned it) with my iPhone and it directed me to: http://ohaipuzzle.com/ which said "SOLVE ME"
Personally I think that QR codes could be used for some fun things, but it is all contingent on the learner having a device that can read the QR code.
I've seen some Microsoft Tag codes, but honestly, they have been few and far between so I haven't bothered downloading the app for my iphone
I did download TagReader a while back (compelled by some pop magazine ad, extra content or contest) and it's flaky -- often does not correctly read the tag/code. Suggestions for reliable readers on the iPhone?
Redlaser works pretty well for me on iOS