Ask HN: What product could be built around NFC and smart tags?

Since the Nexus S is equipped with NFC. I'm curious if anyone has fun/interesting ideas using this technology, other than payment/ticketing. I'm thinking about how to use those smart tags. The functionality is quite similar to QR Codes, except that the user experience is completely different and way more instinctive, and the content on NFC smart tags can be modified.

I know there must be obvious use cases that I'm missing. In any case I'm interested to know if you have thought of interesting ones.