I ran across http://semapedia.org/ a few days ago. It is a very simple concept. It allows you to create QR Code stickers for real-world objects. The codes link to a Wikipedia page. It is pretty much the purest example of what QR Codes can do to connect the real and digital world. Another cool bit: you can see the latest codes that have been generated on the site.

[EDIT: Seems this site is no longer around]